BUG: make CustomBusinessDay respect provided calendar; require numpy.busdaycalendar (#60647)

Author: edzxy

doc/source/whatsnew/v3.0.0.rst

@@ -972,6 +972,7 @@ Datetimelike
 - Bug in constructing arrays with :class:`ArrowDtype` with ``timestamp`` type incorrectly allowing ``Decimal("NaN")`` (:issue:`61773`)
 - Bug in constructing arrays with a timezone-aware :class:`ArrowDtype` from timezone-naive datetime objects incorrectly treating those as UTC times instead of wall times like :class:`DatetimeTZDtype` (:issue:`61775`)
 - Bug in setting scalar values with mismatched resolution into arrays with non-nanosecond ``datetime64``, ``timedelta64`` or :class:`DatetimeTZDtype` incorrectly truncating those scalars (:issue:`56410`)
+- Bug in :class:`CustomBusinessDay` where calendar parameter validation was not enforced, now requires ``numpy.busdaycalendar`` objects (:issue:`60647`)
 
 
 Timedelta

pandas/_libs/tslibs/offsets.pyx

@@ -216,6 +216,16 @@ cdef _get_calendar(weekmask, holidays, calendar):
             pass
         return calendar, holidays
 
+    # GH#60647
+    # Enforce that when a calendar is provided, it must be a numpy.busdaycalendar
+    if calendar is not None and 'pandas_market_calendars' in str(type(calendar)):
+        raise TypeError(
+            "CustomBusinessDay.calendar must be a numpy.busdaycalendar; "
+            f"got {type(calendar).__name__}. "
+            "For pandas_market_calendars please pass the result of "
+            ".holidays() (a DatetimeIndex) instead."
+        )
+
     if holidays is None:
         holidays = []
     try:

pandas/tests/tseries/offsets/test_custom_business_day.py

@@ -82,6 +82,22 @@ def test_calendar(self):
         dt = datetime(2014, 1, 17)
         assert_offset_equal(CDay(calendar=calendar), dt, datetime(2014, 1, 21))
 
+    def test_calendar_validation(self):
+        """Test CustomBusinessDay calendar parameter validation (GH#60647)"""
+        # Valid numpy.busdaycalendar should work
+        calendar = np.busdaycalendar(weekmask="1111100")  # Monday-Friday
+        cbd = CDay(calendar=calendar)
+        assert cbd.calendar is calendar
+
+        # Test error message for pandas_market_calendars scenario
+        class pandas_market_calendars:
+            def __init__(self):
+                self.name = "pandas_market_calendars"
+
+        mock_calendar = pandas_market_calendars()
+        with pytest.raises(TypeError):
+            CDay(calendar=mock_calendar)
+
     def test_roundtrip_pickle(self, offset, offset2):
         def _check_roundtrip(obj):
             unpickled = tm.round_trip_pickle(obj)