GravityView Calendar - Set start day to be Monday for a specific feed

gravityview-calendar-specific-feed-start-day-monday.php

<?php

/**
 * Change the start day to something other than the WordPress settings.
 * 
 * The value must be a number that represents the day of the week.
 * Sunday=0, Monday=1, Tuesday=2, etc.
 *
 * @param array $calendar_options Calendar options
 * @param int   $form_id          Form ID
 * @param int   $feed_id          Calendar feed ID
 */
add_filter( 'gravityview/calendar/options', function( $calendar_options, $form_id, $feed_id ) {
 
	//
	// REPLACE 4 WITH THE ID OF YOUR CALENDAR FEED!
	//
	if( 4 !== $feed_id ) {
		return $calendar_options;
	}
	
	// Set the calendar week to start on Monday
	$calendar_options['firstDay'] = 1;

	return $calendar_options;
});