A today facet that matches month and day without year

<?php

// index date as mmdd for example 1225
add_filter( 'facetwp_index_row', function( $params, $class ) {
	if ( 'today' == $params['facet_name'] ) { // change today to name of your facet
		$raw_value = $params['facet_value'];
		$params['facet_value'] = date( 'md', strtotime( $raw_value ) );
		$params['facet_display_value'] = date( 'F j', strtotime( $raw_value ) );
	}
	return $params;
}, 10, 2 );

// output only the facet choice for today with label of today, if
// there are none, this will be a ghost with show ghosts enabled
add_filter( 'facetwp_facet_render_args', function( $args ) {
    if ( 'today' == $args['facet']['name'] ) {

        $lookup = [];
        $temp_values = [];
        $days = [ date( 'md' ) ];

        foreach ( $args['values'] as $row ) {
            $lookup[ $row['facet_value'] ] = $row;
        }

        foreach ( $days as $day ) {
            if ( isset( $lookup[ $day ] ) ) {
                $temp_values[] = $lookup[ $day ];
            }
            else {
                $temp_values[] = [
                    'facet_value' => $day,
                    'facet_display_value' => 'Today',
                    'counter' => 0
                ];
            }
        }

        $args['values'] = $temp_values;
    }

    return $args;
});