How can I change the day weeks start on in Calendar?

For the GravityCalendar add-on, by default, the week starts on the same day as the value of the "Week Starts On" setting configured in WordPress.

The settings can be accessed by clicking on Settings in the sidebar in the WordPress dashboard, then clicking on "General".

Filters for people comfortable with code:

You can add this snippet to your site to modify the start day to Monday:

add_filter( 'gravityview/calendar/options', function( $calendar_options, $form_id, $feed_id ) {
 
	// Set the calendar week to start on Monday
	$calendar_options['firstDay'] = 1;

	return $calendar_options;
});

If you want to change the starting day, but only for a specific calendar feed, update the code below with the feed ID you're using:

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;
});

Read here how to add these code samples to your website: Where to put code samples.