bfwc_filter_unit_labels.php

<?php
/**
 * Change day into night
 */
function my_bfwc_filter_unit_labels( $labels ) {
	$labels['day']	= array(
		'single' => __( 'Night', 'bfwc' ),
		'plural' => __( 'Nights', 'bfwc' ),
	);
	return $labels;
}
add_filter( 'bfwc_filter_unit_labels', 'my_bfwc_filter_unit_labels' );