[determiner], [pronoun], [adjective], [adverb] [not gradable], nounSarah's having a party to celebrate her thirtieth birthday.The last day in September is the thirtieth.Oh dear, you mean you were/came thirtieth out of thirty.A thirtieth is one of thirty equal parts of something.
THIRTIETH
Meaning of THIRTIETH in English
Cambridge English vocab. Кембриджский английский словарь. 2012