diary-show-holidays-flag is a variable defined in calendar.el.
Its [default] value is t. You can customize this variable.
Documentation:
Non-nil means include holidays in the diary display.
The holidays appear in the mode line of the diary buffer, or in the
fancy diary buffer next to the date. This slows down the diary functions
somewhat; setting it to nil makes the diary display faster.
The variable calendar-holidays can be customized also, to add or remove holidays as needed.