How do precheck some items in multicheck?

I have a multicheck which I populate with the dates of the next 7 days e.g.
25-04-2020 => Saturday 25 April 2020
26-04-2020 => Sunday 26 April 2020
27-04-2020 => Monday 27 April 2020
etc

I would like to always have Monday and Tuesday checked.
How can I do that?
I tried using a ‘default_cb’ callback but didn’t know what to return.

add_action( 'cmb2_admin_init', 'sab_delivery_dates_availability' );
function sab_delivery_dates_availability() {
	$cmb_options = new_cmb2_box( array(
		'id'            => 'delivery_dates',
		'title'         => 'Delivery Dates Availability',
		'object_types' => array( 'options-page' ),
		'option_key'      => 'delivery_dates', // The option key and admin menu page slug.
		'icon_url'        => 'dashicons-calendar-alt', // Menu icon. Only applicable if 'parent_slug' is left empty.
		'capability'      => 'edit_posts', // Capability required to view this options page.
		'position'        => 2, // Menu position. Only applicable if 'parent_slug' is left empty.
		'save_button'     => 'Save Dates',
	) );

	// Create the checkbox list of dates.
	global $default_disable;
	// The index is the date (dd-mm-yyyy format) and the checkbox text is a readable date for useability.
	$today = time();
	$dates_array = array();
	$secs_in_a_day = 60 * 60 * 24;
	for ( $i = 0; $i < 7; $i++ ) {
		$time = $today + ( $i * $secs_in_a_day );  // Add $i days to time.
		$date_info = getdate( $time );
		// Index is dd-mm-yyyy (for DatePicker date exclusion) and then the readable date.
		$date_index = sprintf( '%02d-%02d-%d', $date_info['mday'], $date_info['mon'], $date_info['year'] );
		$dates_array[ $date_index ] = sprintf( '%s %d %s %d', $date_info['weekday'], $date_info['mday'], $date_info['month'], $date_info['year'] );
	}

	$cmb_options->add_field( array(
		'desc' => 'Check dates that do <strong>NOT</strong> have delivery slots available.',
		'type' => 'multicheck',
		'id' => 'dates',
		'select_all_button' => false,  // Exclude the "Select All" button.
		'options' => $dates_array,
	) );
}