code: def test_dti_is_month_start_custom(self):
      # Ensure is_start/end accessors throw ValueError for CustomBusinessDay,
      bday_egypt = offsets.CustomBusinessDay(
          weekmask="Sun Mon Tue Wed Thu"
      )
      dti = date_range(
          datetime(2013, 4, 30), periods=5, freq=bday_egypt
      )
      msg = "Custom business days is not supported by is_month_start"
      with pytest.raises(ValueError, match=msg):
          dti.is_month_start
  
imports: import calendar
  from datetime import date, datetime, time
  import locale
  import unicodedata
  import numpy
  import pytest
  from pandas._libs.tslibs import timezones
  from pandas import DatetimeIndex, Index, NaT, Timestamp, date_range, offsets
  import pandas._testing
  from pandas.core.arrays import DatetimeArray
label_desc: Use image node_id 15 for calling the TestDatetimeIndexOps obj's underlying member method code with example usage: obj.test_dti_is_month_start_custom() without return types