How do you know that the holidays are correct when the calendar has more than
3 months? Do you have a way to check, say, the dates of lunar phases, Easter,
Chinese New Year, Yom Kippur, Islamic New Year, solstices/equinoxes? You will
see SOME holidays (or diary entries) but not all that apply to the months
displayed.

One easy way to check is to run the standard calendar code (without
this customization) and see if it indicates the same holidays.