function month_multiday_classes( $event, $day_date, $is_start_of_week, $today_date ) {
	$classes = tribe_get_post_class( [ 'tribe-events-calendar-month__multiday-event' ], $event->ID );

	if ( ! empty( $event->featured ) ) {
		$classes[] = 'tribe-events-calendar-month__multiday-event--featured';
	}

	/*
	 * To keep the calendar accessible, in the context of a week, we'll print the event only on either its first day
	 * or the first day of the week.
	 */
	$should_display = $is_start_of_week || in_array( $day_date, $event->displays_on, true );

	// If doesn't start today and this week, let's not add the left border or set the width.
	if ( ! $should_display ) {
		/**
		 * Allows filtering the multiday event classes.
		 *
		 * @since 5.1.1
		 *
		 * @param array<string> $classes    An array of thee classes to be applied.
		 * @param WP_Post $event            An event post object with event-specific properties added from the the `tribe_get_event`
		 *                                  function.
		 * @param string  $day_date         The `Y-m-d` date of the day currently being displayed.
		 * @param bool    $is_start_of_week Whether the current grid day being rendered is the first day of the week or not.
		 * @param string  $today_date       Today's date in the `Y-m-d` format.
		 */
		return apply_filters( 'tribe_events_views_v2_month_multiday_classes', $classes, $event, $day_date, $is_start_of_week, $today_date );
	}

	/*
	* The "duration" here is how many days the event will take this week, not in total.
	* The two values might be the same but they will differ for events that last more than one week.
	*/
	$classes[] = 'tribe-events-calendar-month__multiday-event--width-' . $event->this_week_duration;
	$classes[] = 'tribe-events-calendar-month__multiday-event--display';

	// If it ends this week, let's add the start class (left border).
	if ( $event->starts_this_week ) {
		$classes[] = 'tribe-events-calendar-month__multiday-event--start';
	}

	// If it ends this week, let's add the end class (right border).
	if ( $event->ends_this_week ) {
		$classes[] = 'tribe-events-calendar-month__multiday-event--end';
	}

	if ( $event->dates->end->format( 'Y-m-d' ) < $today_date ) {
		$classes[] = 'tribe-events-calendar-month__multiday-event--past';
	}

	/**
	 * Allows filtering the multiday event classes.
	 *
	 * @since 5.1.1
	 *
	 * @param array<string> $classes    An array of thee classes to be applied.
	 * @param WP_Post $event            An event post object with event-specific properties added from the the `tribe_get_event`
	 *                                  function.
	 * @param string  $day_date         The `Y-m-d` date of the day currently being displayed.
	 * @param bool    $is_start_of_week Whether the current grid day being rendered is the first day of the week or not.
	 * @param string  $today_date       Today's date in the `Y-m-d` format.
	 */
	return apply_filters( 'tribe_events_views_v2_month_multiday_classes', $classes, $event, $day_date, $is_start_of_week, $today_date );
}