This example will Blackout the New Years day.

add_filter( 'frm_date_field_options', 'add_blackout_dates', 30, 2 );
function add_blackout_dates( $js_options, $extra ) {
	if ( $extra['field_id'] === 'field_jtumj' ) { // Replace jtumj with your field key
		$js_options['formidable_dates']['datesDisabled'][] = date( 'Y-m-d', strtotime( 'first day of january' ) );
	}

	return $js_options;
}

By default, the month and year are displayed in dropdown lists. By using this code, it will allow you to show the month and year in the calendar display header and have users select dates by using the next/previous buttons.

add_filter( 'frm_date_field_options', 'hide_month_and_year', 30, 2 );
function hide_month_and_year( $js_options, $extra ) {
	if ( $extra['field_id'] === 'field_jtumj' ) { // Replace jtumj with your field key
		$js_options['options']['changeMonth'] = false;
		$js_options['options']['changeYear'] = false;
	}

	return $js_options;
}

Use this code example to set the first day of the week in the datepicker for all forms. For example, if you want to set the first day of the week to Sunday, replace the value of 3 with 7.

add_filter('frm_date_field_options', 'set_start_day_all' );
function set_start_day_all( $options ) {
	$options['options']['firstDay'] = 3;
	return $options;
}

Use this code example to set the first day of the week in the datepicker for a specific date field key.

add_filter('frm_date_field_options', 'set_start_day', 10, 2);
function set_start_day( $options, $args ) {
	$target_field_key = 'lyy3w'; // Replace lyy3w with the key of the date field.
	if ( 'field_' . $target_field_key === $args['field_id'] ) {
		$options['options']['firstDay'] = 3;
	}
	return $options;
}