diff --git a/pandas/_typing.py b/pandas/_typing.py
index a9852dd4b13cf..21ea5ce5e4a83 100644
--- a/pandas/_typing.py
+++ b/pandas/_typing.py
@@ -21,6 +21,7 @@
     Dict,
     Hashable,
     List,
+    Literal,
     Mapping,
     Optional,
     Sequence,
@@ -37,7 +38,6 @@
 # https://mypy.readthedocs.io/en/latest/common_issues.html#import-cycles
 if TYPE_CHECKING:
     from typing import (
-        Literal,
         TypedDict,
         final,
     )
@@ -123,6 +123,8 @@
 Frequency = Union[str, "DateOffset"]
 Axes = Collection[Any]
 RandomState = Union[int, ArrayLike, np.random.Generator, np.random.RandomState]
+MergeTypes = Literal["inner", "outer", "left", "right", "cross"]
+ConcatTypes = Literal["inner", "outer"]
 
 # dtypes
 NpDtype = Union[str, np.dtype]
diff --git a/pandas/core/apply.py b/pandas/core/apply.py
index 69e2650a15f16..7f7f59615e8df 100644
--- a/pandas/core/apply.py
+++ b/pandas/core/apply.py
@@ -9,6 +9,7 @@
     Hashable,
     Iterator,
     List,
+    Literal,
     cast,
 )
 import warnings
@@ -518,7 +519,10 @@ def apply_multiple(self) -> FrameOrSeriesUnion:
         return self.obj.aggregate(self.f, self.axis, *self.args, **self.kwargs)
 
     def normalize_dictlike_arg(
-        self, how: str, obj: FrameOrSeriesUnion, func: AggFuncTypeDict
+        self,
+        how: Literal["apply", "agg", "transform"],
+        obj: FrameOrSeriesUnion,
+        func: AggFuncTypeDict,
     ) -> AggFuncTypeDict:
         """
         Handler for dict-like argument.
@@ -527,7 +531,10 @@ def normalize_dictlike_arg(
         that a nested renamer is not passed. Also normalizes to all lists
         when values consists of a mix of list and non-lists.
         """
-        assert how in ("apply", "agg", "transform")
+        if how not in ("apply", "agg", "transform"):
+            raise ValueError(
+                "Value for how argument must be one of : apply, agg, transform"
+            )
 
         # Can't use func.values(); wouldn't work for a Series
         if (
diff --git a/pandas/core/arrays/_ranges.py b/pandas/core/arrays/_ranges.py
index 3909875e5660a..3018219219e58 100644
--- a/pandas/core/arrays/_ranges.py
+++ b/pandas/core/arrays/_ranges.py
@@ -4,6 +4,8 @@
 """
 from __future__ import annotations
 
+from typing import Literal
+
 import numpy as np
 
 from pandas._libs.lib import i8max
@@ -75,7 +77,7 @@ def generate_regular_range(
 
 
 def _generate_range_overflow_safe(
-    endpoint: int, periods: int, stride: int, side: str = "start"
+    endpoint: int, periods: int, stride: int, side: Literal["start", "end"] = "start"
 ) -> int:
     """
     Calculate the second endpoint for passing to np.arange, checking
@@ -142,13 +144,14 @@ def _generate_range_overflow_safe(
 
 
 def _generate_range_overflow_safe_signed(
-    endpoint: int, periods: int, stride: int, side: str
+    endpoint: int, periods: int, stride: int, side: Literal["start", "end"]
 ) -> int:
     """
     A special case for _generate_range_overflow_safe where `periods * stride`
     can be calculated without overflowing int64 bounds.
     """
     assert side in ["start", "end"]
+
     if side == "end":
         stride *= -1
 
diff --git a/pandas/core/arrays/datetimes.py b/pandas/core/arrays/datetimes.py
index 92a906e9fd8b0..62554a067d1a0 100644
--- a/pandas/core/arrays/datetimes.py
+++ b/pandas/core/arrays/datetimes.py
@@ -37,6 +37,7 @@
     to_offset,
     tzconversion,
 )
+from pandas._typing import Dtype
 from pandas.errors import PerformanceWarning
 
 from pandas.core.dtypes.cast import astype_dt64_to_dt64tz
@@ -1967,12 +1968,12 @@ def sequence_to_datetimes(
 
 def sequence_to_dt64ns(
     data,
-    dtype=None,
-    copy=False,
-    tz=None,
-    dayfirst=False,
-    yearfirst=False,
-    ambiguous="raise",
+    dtype: Dtype | None = None,
+    copy: bool = False,
+    tz: tzinfo | str = None,
+    dayfirst: bool = False,
+    yearfirst: bool = False,
+    ambiguous: str | bool = "raise",
     *,
     allow_object: bool = False,
     allow_mixed: bool = False,
@@ -2126,10 +2127,10 @@ def sequence_to_dt64ns(
 
 def objects_to_datetime64ns(
     data: np.ndarray,
-    dayfirst,
-    yearfirst,
-    utc=False,
-    errors="raise",
+    dayfirst: bool,
+    yearfirst: bool,
+    utc: bool = False,
+    errors: Literal["raise", "coerce", "ignore"] = "raise",
     require_iso8601: bool = False,
     allow_object: bool = False,
     allow_mixed: bool = False,
@@ -2164,7 +2165,10 @@ def objects_to_datetime64ns(
     ------
     ValueError : if data cannot be converted to datetimes
     """
-    assert errors in ["raise", "ignore", "coerce"]
+    if errors not in ["raise", "ignore", "coerce"]:
+        raise ValueError(
+            "Value for errors argument must be one of: raise, coerce, ignore"
+        )
 
     # if str-dtype, convert
     data = np.array(data, copy=False, dtype=np.object_)
diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 0acbb0c34266f..d2d15d3cbfe8b 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -9191,7 +9191,7 @@ def merge(
         sort: bool = False,
         suffixes: Suffixes = ("_x", "_y"),
         copy: bool = True,
-        indicator: bool = False,
+        indicator: bool | str = False,
         validate: str | None = None,
     ) -> DataFrame:
         from pandas.core.reshape.merge import merge
diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index f3063d9d71b2c..fe01335fac1bc 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -5691,7 +5691,9 @@ def _validate_indexer(self, form: str_t, key, kind: str_t):
         if key is not None and not is_integer(key):
             raise self._invalid_indexer(form, key)
 
-    def _maybe_cast_slice_bound(self, label, side: str_t, kind=no_default):
+    def _maybe_cast_slice_bound(
+            self, label, side: str_t, kind: Literal["loc", "getitem"] = no_default
+    ):
         """
         This function should be overloaded in subclasses that allow non-trivial
         casting on label-slice bounds, e.g. datetime-like indices allowing
@@ -5755,7 +5757,10 @@ def get_slice_bound(self, label, side: str_t, kind=None) -> int:
         int
             Index of label.
         """
-        assert kind in ["loc", "getitem", None]
+        if kind not in ["loc", "getitem", None]:
+            raise ValueError(
+                "Value for kind argument must be one of: loc, getitem or None"
+            )
 
         if side not in ("left", "right"):
             raise ValueError(
diff --git a/pandas/core/internals/blocks.py b/pandas/core/internals/blocks.py
index 6275fe39558a3..35d85ebd0d974 100644
--- a/pandas/core/internals/blocks.py
+++ b/pandas/core/internals/blocks.py
@@ -1200,7 +1200,8 @@ def where(self, other, cond, errors="raise") -> list[Block]:
         assert cond.ndim == self.ndim
         assert not isinstance(other, (ABCIndex, ABCSeries, ABCDataFrame))
 
-        assert errors in ["raise", "ignore"]
+        if errors not in ["raise", "ignore"]:
+            raise ValueError("Value for errors argument must be one of: raise, ignore")
         transpose = self.ndim == 2
 
         values = self.values
diff --git a/pandas/core/missing.py b/pandas/core/missing.py
index f144821220e4b..0fd84494a4681 100644
--- a/pandas/core/missing.py
+++ b/pandas/core/missing.py
@@ -10,6 +10,7 @@
 from typing import (
     TYPE_CHECKING,
     Any,
+    Literal,
     cast,
 )
 
@@ -164,7 +165,7 @@ def clean_interp_method(method: str, index: Index, **kwargs) -> str:
     return method
 
 
-def find_valid_index(values, *, how: str) -> int | None:
+def find_valid_index(values, *, how: Literal["first", "last"]) -> int | None:
     """
     Retrieves the index of the first valid value.
 
@@ -178,7 +179,8 @@ def find_valid_index(values, *, how: str) -> int | None:
     -------
     int or None
     """
-    assert how in ["first", "last"]
+    if how not in ["first", "last"]:
+        raise ValueError("Value for how argument must be one of : first, last")
 
     if len(values) == 0:  # early stop
         return None
diff --git a/pandas/core/reshape/merge.py b/pandas/core/reshape/merge.py
index 143999a4677b3..8a3c6985b1f08 100644
--- a/pandas/core/reshape/merge.py
+++ b/pandas/core/reshape/merge.py
@@ -28,7 +28,9 @@
     DtypeObj,
     FrameOrSeries,
     IndexLabel,
+    MergeTypes,
     Suffixes,
+    TimedeltaConvertibleTypes,
 )
 from pandas.errors import MergeError
 from pandas.util._decorators import (
@@ -92,7 +94,7 @@
 def merge(
     left: DataFrame | Series,
     right: DataFrame | Series,
-    how: str = "inner",
+    how: MergeTypes = "inner",
     on: IndexLabel | None = None,
     left_on: IndexLabel | None = None,
     right_on: IndexLabel | None = None,
@@ -101,7 +103,7 @@ def merge(
     sort: bool = False,
     suffixes: Suffixes = ("_x", "_y"),
     copy: bool = True,
-    indicator: bool = False,
+    indicator: bool | str = False,
     validate: str | None = None,
 ) -> DataFrame:
     op = _MergeOperation(
@@ -331,11 +333,11 @@ def merge_asof(
     right_on: IndexLabel | None = None,
     left_index: bool = False,
     right_index: bool = False,
-    by=None,
-    left_by=None,
-    right_by=None,
+    by: IndexLabel | None = None,
+    left_by: Hashable | None = None,
+    right_by: Hashable | None = None,
     suffixes: Suffixes = ("_x", "_y"),
-    tolerance=None,
+    tolerance: None | TimedeltaConvertibleTypes = None,
     allow_exact_matches: bool = True,
     direction: str = "backward",
 ) -> DataFrame:
@@ -622,7 +624,7 @@ def __init__(
         sort: bool = True,
         suffixes: Suffixes = ("_x", "_y"),
         copy: bool = True,
-        indicator: bool = False,
+        indicator: bool | str = False,
         validate: str | None = None,
     ):
         _left = _validate_operand(left)
diff --git a/pandas/io/excel/_util.py b/pandas/io/excel/_util.py
index 7d8028de23257..d8b2a58218f58 100644
--- a/pandas/io/excel/_util.py
+++ b/pandas/io/excel/_util.py
@@ -59,7 +59,8 @@ def get_default_engine(ext, mode="reader"):
         "xls": "xlwt",
         "ods": "odf",
     }
-    assert mode in ["reader", "writer"]
+    if mode not in ["reader", "writer"]:
+        raise ValueError('File mode must be either "reader" or "writer".')
     if mode == "writer":
         # Prefer xlsxwriter over openpyxl if installed
         xlsxwriter = import_optional_dependency("xlsxwriter", errors="warn")
diff --git a/pandas/tseries/frequencies.py b/pandas/tseries/frequencies.py
index c55ac6ce228bf..7b84dbb635133 100644
--- a/pandas/tseries/frequencies.py
+++ b/pandas/tseries/frequencies.py
@@ -445,7 +445,7 @@ def _maybe_add_count(base: str, count: float) -> str:
 # Frequency comparison
 
 
-def is_subperiod(source, target) -> bool:
+def is_subperiod(source: str | DateOffset, target: str | DateOffset) -> bool:
     """
     Returns True if downsampling is possible between source and target
     frequencies
@@ -501,7 +501,7 @@ def is_subperiod(source, target) -> bool:
         return False
 
 
-def is_superperiod(source, target) -> bool:
+def is_superperiod(source: str | DateOffset, target: str | DateOffset) -> bool:
     """
     Returns True if upsampling is possible between source and target
     frequencies
@@ -559,7 +559,7 @@ def is_superperiod(source, target) -> bool:
         return False
 
 
-def _maybe_coerce_freq(code) -> str:
+def _maybe_coerce_freq(code: str | DateOffset) -> str:
     """we might need to coerce a code to a rule_code
     and uppercase it
 
diff --git a/pandas/tseries/holiday.py b/pandas/tseries/holiday.py
index 54ac116afe3cf..165f3fcb62bd5 100644
--- a/pandas/tseries/holiday.py
+++ b/pandas/tseries/holiday.py
@@ -4,6 +4,11 @@
     datetime,
     timedelta,
 )
+from typing import (
+    Any,
+    Callable,
+    Tuple,
+)
 import warnings
 
 from dateutil.relativedelta import (  # noqa
@@ -33,6 +38,8 @@
     Easter,
 )
 
+from pandas._typing import TimestampConvertibleTypes
+
 
 def next_monday(dt: datetime) -> datetime:
     """
@@ -151,27 +158,39 @@ class Holiday:
 
     def __init__(
         self,
-        name,
-        year=None,
-        month=None,
-        day=None,
-        offset=None,
-        observance=None,
-        start_date=None,
-        end_date=None,
-        days_of_week=None,
+        name: str,
+        year: int | None = None,
+        month: int | None = None,
+        day: int | None = None,
+        offset: list[Any] | Any = None,
+        observance: Callable[..., DatetimeIndex] = None,
+        start_date: TimestampConvertibleTypes = None,
+        end_date: TimestampConvertibleTypes = None,
+        days_of_week: tuple[int, ...] = None,
     ):
         """
         Parameters
         ----------
         name : str
             Name of the holiday , defaults to class name
+        year : int, optional
+            the year in which the holiday occurs
+        month : int, optional
+            the month in which the holiday occurs
+        day : int, optional
+             the day on which the holiday occurs
         offset : array of pandas.tseries.offsets or
                 class from pandas.tseries.offsets
             computes offset from date
         observance: function
             computes when holiday is given a pandas Timestamp
-        days_of_week:
+        start_date : datetime-like
+            optionally constrain the period in which the holdiday occurs
+            can be any valid value for a pandas Timestamp
+        end_date : datetime-like
+            optionally constrain the period in which the holdiday occurs
+            can be any valid value for a pandas Timestamp
+        days_of_week: tuple(int, ...)
             provide a tuple of days e.g  (0,1,2,3,) for Monday Through Thursday
             Monday=0,..,Sunday=6
 
@@ -239,7 +258,12 @@ def __repr__(self) -> str:
         repr = f"Holiday: {self.name} ({info})"
         return repr
 
-    def dates(self, start_date, end_date, return_name=False):
+    def dates(
+        self,
+        start_date: TimestampConvertibleTypes,
+        end_date: TimestampConvertibleTypes,
+        return_name: bool = False,
+    ):
         """
         Calculate holidays observed between start date and end date
 
@@ -286,7 +310,9 @@ def dates(self, start_date, end_date, return_name=False):
             return Series(self.name, index=holiday_dates)
         return holiday_dates
 
-    def _reference_dates(self, start_date, end_date):
+    def _reference_dates(
+        self, start_date: TimestampConvertibleTypes, end_date: TimestampConvertibleTypes
+    ):
         """
         Get reference dates for the holiday.
 
@@ -319,7 +345,7 @@ def _reference_dates(self, start_date, end_date):
 
         return dates
 
-    def _apply_rule(self, dates):
+    def _apply_rule(self, dates: DatetimeIndex):
         """
         Apply the given offset/observance to a DatetimeIndex of dates.
 
@@ -390,7 +416,7 @@ class AbstractHolidayCalendar(metaclass=HolidayCalendarMetaClass):
     end_date = Timestamp(datetime(2200, 12, 31))
     _cache = None
 
-    def __init__(self, name=None, rules=None):
+    def __init__(self, name: str = None, rules: list[Holiday] = None):
         """
         Initializes holiday object with a given set a rules.  Normally
         classes just have the rules defined within them.
@@ -417,7 +443,12 @@ def rule_from_name(self, name):
 
         return None
 
-    def holidays(self, start=None, end=None, return_name=False):
+    def holidays(
+        self,
+        start: TimestampConvertibleTypes = None,
+        end: TimestampConvertibleTypes = None,
+        return_name: bool = False
+    ) -> DatetimeIndex:
         """
         Returns a curve with holidays between start_date and end_date