From 34177d6b20896bf101590306e3f0e1e0a225c5fb Mon Sep 17 00:00:00 2001
From: Natalia Mokeeva <91160475+natmokval@users.noreply.github.com>
Date: Sat, 11 May 2024 20:17:42 +0200
Subject: [PATCH] BUG: DatetimeIndex.is_year_start breaks on custom business
 days frequencies bigger then `1C` (#58665)

* fix bug dti.is_year_start breaks on freq custom business day with digit

* rename the variable freqstr

* delete double space
---
 doc/source/whatsnew/v3.0.0.rst                       | 3 ++-
 pandas/_libs/tslibs/fields.pyx                       | 2 +-
 pandas/tests/indexes/datetimes/test_scalar_compat.py | 7 +++++++
 3 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/doc/source/whatsnew/v3.0.0.rst b/doc/source/whatsnew/v3.0.0.rst
index ade659d6e9e34..08b3175ad0ad6 100644
--- a/doc/source/whatsnew/v3.0.0.rst
+++ b/doc/source/whatsnew/v3.0.0.rst
@@ -384,6 +384,8 @@ Datetimelike
 - Bug in :func:`date_range` where the last valid timestamp would sometimes not be produced (:issue:`56134`)
 - Bug in :func:`date_range` where using a negative frequency value would not include all points between the start and end values (:issue:`56382`)
 - Bug in :func:`tseries.api.guess_datetime_format` would fail to infer time format when "%Y" == "%H%M" (:issue:`57452`)
+- Bug in :meth:`DatetimeIndex.is_year_start` and :meth:`DatetimeIndex.is_quarter_start` does not raise on Custom business days frequencies bigger then "1C" (:issue:`58664`)
+- Bug in :meth:`DatetimeIndex.is_year_start` and :meth:`DatetimeIndex.is_quarter_start` returning ``False`` on double-digit frequencies (:issue:`58523`)
 - Bug in setting scalar values with mismatched resolution into arrays with non-nanosecond ``datetime64``, ``timedelta64`` or :class:`DatetimeTZDtype` incorrectly truncating those scalars (:issue:`56410`)
 
 Timedelta
@@ -421,7 +423,6 @@ Interval
 Indexing
 ^^^^^^^^
 - Bug in :meth:`DataFrame.__getitem__` returning modified columns when called with ``slice`` in Python 3.12 (:issue:`57500`)
-- Bug in :meth:`DatetimeIndex.is_year_start` and :meth:`DatetimeIndex.is_quarter_start` returning ``False`` on double-digit frequencies (:issue:`58523`)
 -
 
 Missing
diff --git a/pandas/_libs/tslibs/fields.pyx b/pandas/_libs/tslibs/fields.pyx
index 8f8060b2a5f83..399a5c2e96cd5 100644
--- a/pandas/_libs/tslibs/fields.pyx
+++ b/pandas/_libs/tslibs/fields.pyx
@@ -253,7 +253,7 @@ def get_start_end_field(
         # month of year. Other offsets use month, startingMonth as ending
         # month of year.
 
-        if freq_name.lstrip("B")[0:2]  in ["MS", "QS", "YS"]:
+        if freq_name.lstrip("B")[0:2] in ["MS", "QS", "YS"]:
             end_month = 12 if month_kw == 1 else month_kw - 1
             start_month = month_kw
         else:
diff --git a/pandas/tests/indexes/datetimes/test_scalar_compat.py b/pandas/tests/indexes/datetimes/test_scalar_compat.py
index 1f2cfe8b7ddc8..5831846c9ceb6 100644
--- a/pandas/tests/indexes/datetimes/test_scalar_compat.py
+++ b/pandas/tests/indexes/datetimes/test_scalar_compat.py
@@ -336,3 +336,10 @@ def test_dti_is_year_quarter_start_doubledigit_freq(self):
 
         dr = date_range("2017-01-01", periods=2, freq="10QS")
         assert all(dr.is_quarter_start)
+
+    def test_dti_is_year_start_freq_custom_business_day_with_digit(self):
+        # GH#58664
+        dr = date_range("2020-01-01", periods=2, freq="2C")
+        msg = "Custom business days is not supported by is_year_start"
+        with pytest.raises(ValueError, match=msg):
+            dr.is_year_start