The Events Calendar - Filter Bar -- make Event Categories filter by AND (in all selected categories) instead of OR (in any selected category, which is the default)

functions.php

<?php

/**
 * !!!
 * Old snippet. Use the tribe_events_filter_taxonomy_relationship filter instead.
 * https://gist.github.com/cliffordp/c2ea6115318044db219322fea8da55ec
 * !!!
 *
 * By Barry 2016-07-06
 * From https://gist.github.com/cliffordp/eb2753491526b079ba69882f4be4fafc
 *
 * See https://gist.github.com/niconerd/e5b624a9e85e0895ff4393354e4d2ef0 for similar solution but for Tags
 *
 * Attempts to modify the taxonomy query added by Filter Bar's
 * category filter to use AND type logic instead of OR.
 *
 * For example, by default, if Categories A and B are selected
 * using Filter Bar it will return events in EITHER of those
 * categories. With this modification in place, however, it
 * will whittle things down to only events that are assigned
 * to BOTH those terms.
 *
 * Takes a slightly shotgun approach, would need refinement in
 * anything beyond very simple scenarios.
 */
class Category_Filter__Use_And_Logic {
    private static $engage = false;

    static function init() {
        add_action( 'tribe_events_filter_taxonomy_relationship', __CLASS__ . '::listen' );
        add_action( 'tribe_events_pre_get_posts', __CLASS__ . '::change', 100 );
    }

    static function listen() {
        self::$engage = true;
    }

    static function change( $query ) {
        if ( ! self::$engage ) return;

        $tax_query = $query->get( 'tax_query' );
        if ( ! is_array( $tax_query ) || ! isset( $tax_query[ 'Tribe__Events__Filterbar__Filters__Category' ] ) ) return;

        foreach ( $tax_query[ 'Tribe__Events__Filterbar__Filters__Category' ] as &$inner_query )
            if ( is_array( $inner_query ) ) $inner_query['operator'] = 'AND';

        $query->set( 'tax_query', $tax_query );
    }
}

/* Enable AND type logic for category filters.
 *
 * Comment out or delete the following line
 * to temporarily disable this mod.
 */
Category_Filter__Use_And_Logic::init();