prefix_date_field_params_disable_weekends_holidays.php

<?php
/**
 * Disable weekends and holidays
 */
function prefix_date_field_params_disable_weekends_holidays( $params, $item ) {
	$params[] = 'beforeShowDay : function( date ) {
		// Disable holidays
		var holidays = ["2020-04-13","2020-04-14"];
		var datestring = jQuery.datepicker.formatDate( "yy-mm-dd", date );
		
		if( holidays.indexOf( datestring ) != -1 ) {
			return [ false, false ];
		}

		// Disable weekends
		var day = date.getDay();
		if( day == 0 || day == 6 ) {
			return [ false, false ];
		}
		return [ true ];
	}';
	return $params;
}
add_filter( 'pewc_filter_date_field_params', 'prefix_date_field_params_disable_weekends_holidays', 10, 2 );