Custom Day of Week filter for "The Events Calendar" plugin that filters events that only start on the selected dates.

class-theme-day-of-week-simple-filter.php

<?php

use Tribe\Events\Filterbar\Views\V2\Filters\Day_Of_Week;

/**
 * Class Theme_Day_Of_Week_Simple_Filter
 *
 * @see \Tribe\Events\Filterbar\Views\V2\Filters\Day_Of_Week
 * @see \Tribe__Events__Filterbar__Filters__Day_Of_Week
 */
class Theme_Day_Of_Week_Simple_Filter extends Day_Of_Week {
	/**
	 * Updates the where clause to only add a simple DAYOFWEEK() check.
	 *
	 * @return void
	 */
	protected function setup_where_clause() {
		/** @var wpdb $wpdb */
		global $wpdb;

		$values = array_map( 'intval', $this->currentValue );
		$values = implode( ',', $values );

		$eod_cutoff = tribe_get_option( 'multiDayCutoff', '00:00' );
		if ( $eod_cutoff != '00:00' ) {
			$eod_time_difference = Tribe__Date_Utils::time_between( '1/1/2014 00:00:00', "1/1/2014 {$eod_cutoff}:00" );
			$start_date = "DATE_SUB({$wpdb->postmeta}.meta_value, INTERVAL {$eod_time_difference} SECOND)";
		} else {
			$start_date = "{$wpdb->postmeta}.meta_value";
		}

		$this->whereClause = " AND (DAYOFWEEK($start_date) IN ($values))";

		// Forces the query to use _EventStartDate and _EventEndDate as the times to base results
		// off of, instead of _EventStartDateUTC, _EventEventDateUTC which can produce weird results.
		add_filter( 'tribe_events_query_force_local_tz', '__return_true' );
	}
}

tribe-events-day-of-week-filter.php

<?php

require_once __DIR__ . '/class-theme-day-of-week-simple-filter.php';

/**
 * Custom Day of Week filter for The Events Calendar plugin that filters events that only start on
 * the selected dates.
 *
 * New filter available in WP-Admin > Events > Settings > Filters
 */

/**
 * Includes the custom filter class and creates an instance of it.
 */
add_action( 'tribe_events_filters_create_filters', function() {
	new \Theme_Day_Of_Week_Simple_Filter( 'Weekday', 'filterbar_day_of_week_simple' );
} );

/**
 * Filters the Context locations to let the Context know how to fetch the value of the filter from
 * a request.
 *
 * @param array<string,array> $locations A map of the locations the Context supports and is able to
 *                                       read from and write to.
 *
 * @return array<string,array> The filtered map of Context locations, with the one required from
 *                             the filter added to it.
 */
add_filter( 'tribe_context_locations', function( array $locations ) {
	// Read the filter selected values, if any, from the URL request vars.
	$locations['filterbar_day_of_week_simple'] = [
		'read' => [
			\Tribe__Context::REQUEST_VAR => [ 'tribe_filterbar_day_of_week_simple' ],
		],
	];

	return $locations;
} );

/**
 * Filters the map of filters available on the front-end to include the custom one.
 *
 * @param array<string,string> $map A map relating the filter slugs to their respective classes.
 *
 * @return array<string,string> The filtered slug to filter class map.
 */
add_filter( 'tribe_events_filter_bar_context_to_filter_map', function( array $map ) {
	$map['filterbar_day_of_week_simple'] = Theme_Day_Of_Week_Simple_Filter::class;

	return $map;
} );