I want to show an error if it's not a weekday (you can enter a date in the html section)
setlocale(LC_TIME, "de_DE.utf8");
$dateString = strftime('%A' , $dateTimestamp1);
substr($dateString, 0, 2);
if ($dateString == 'So' || $dateString == 'Sa') {
throw new FormInputException('date', 'Invalid Date');
}