Meaning of -TH in English
BAD : It was their 23th wedding anniversary.
GOOD : It was their 23rd wedding anniversary.
Use -th when the word for the number ends with '-th': '6th May' (= six th ), '18th May' (= eighteen th ). Do not use -th with (numbers ending) 1, 2, 3.
Compare: '1st May' (fir st ), '2nd May ' (seco nd ), '3rd May ' (thi rd )
Longman Common Errors English vocabulary. Английский словарь распространенных ошибок Longman. 2012