IF field conditionned by time/date field

We have a lot of letter templates to use in official functions. All are in Luxembourgish language. We use a time/date field (not having the right date in those letters is embarassing).

Format is "Lëtzebuerg, den <date>"

Problem is that in Luxembourgish language the article ("den") becomes "de" when placed before certain sounds. I.g. you would write "den 13. Dezember" but "de 14. Dezember".

So I'd need an IF datefield starts with 4 OR 5 OR 6 OR 7 OR 14. OR 15 OR 16 OR 17 OR 24 OR25 OR 26 OR 27 "de" "den"

Is this possible? Do Boolean operators work inside fields? Do I have to make a separate IF-Field for each number? Can I get help here, please?