This example blacks out the next New Year's Day. It works with the Flatpickr date picker library, the default since Formidable Forms 7.0. With the jQuery date picker library, it only works when the Datepicker Options add-on is active.

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 next year' ) );
	}

	return $js_options;
}

By default, users pick the month from a dropdown list in the calendar header and change the year in the box next to it. With the jQuery date picker library, the year is a dropdown list too.

Use this code to show the month as text in the calendar header. How users change the month depends on the date picker library, as described below the code.

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;
}

With the jQuery date picker library, this code shows the month and year as text, with the previous and next arrows.

With the Flatpickr date picker library, the month shows as text and the year box stays. The previous and next arrows are hidden. To show the arrows, add this CSS in Formidable → Global Settings → Custom CSS:

.frm-date-no-month-select .flatpickr-prev-month,
.frm-date-no-month-select .flatpickr-next-month {
	display: flex !important;
}

Use this code example to set the first day of the week in the datepicker for all forms. The value is the day number: 0 is Sunday, 1 is Monday, and so on up to 6 for Saturday. For example, if you want to set the first day of the week to Sunday, replace the value of 3 with 0.

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;
}