wpcrm_system_add_calendar_entry

functions.php

function wpcrm_system_new_calendar_entry( $calendar, $month, $list_day, $year ){
	// Display an entry on the calender on December 25th every year.
	if ( $month == 12 && $list_day == 25 ){
		$calendar .= '<li>Christmas</li>';
	}
	// Display an entry on the calendar on September 5th 2017
	if ( $month == 9 && $list_day == 5 && $year == 2017 ){
		$calendar .= '<li>This is a date specific entry</li>';
	}
	// Display an entry on the calendar on the 15th of every month
	if ( $list_day == 15 ){
		$calendar .= '<li>This is an entry for the 15th of every month</li>';
	}
	return $calendar;
}
add_filter( 'wpcrm_system_add_calendar_entry', 'wpcrm_system_new_calendar_entry', 10, 4 );