@Silvio6 Since there are no functions available to directly get first or last day of the month you can try something like this:
Promotion valid from "01/{% let d=now %}{%= formatDate(d, "MM") %}/{%= formatDate(d, "YYYY") %}" to "{% let lastday= setDays(addMonths(d, 1), 1)%} {% let a = addDays(lastday, -1) %}{%= formatDate(a, "dd") %}/{%= formatDate(a, "MM") %}/{%= formatDate(a, "YYYY") %}"
Hope this helps.