Correct spelling, explanation: 25th is an ordinal number and its full form is spelled twenty-fifth. It appears between twenty-fourth and twenty-sixth. It’s one of the regular ordinal numbers that are formed by adding -th suffix to their roots. How do we know that it’s regular? Because it ends with five, not one, two, or three. The first three numbers are formed into ordinal numbers differently (one changes to first, two changes to second, and three changes to third). The -st suffix is an abbreviation of first, but this word doesn’t appear in twenty-fifth. The only correct form is 25th, not 25st.

Definition of 25th:
numeral, coming next after twenty-fourth in position
I was born on 25th July 1997 in Warsaw.
I threw a huge party to celebrate my 25th birthday.

Collocations with 25th:
Some most commonly used collocations include: 25th birthday, and 25th day of a month.

Incorrect spelling, explanation: this mistake may come from the fact that different ordinal numbers have different suffixes: -th, -st, -nd, or -rd. It all depends on what number is on their end. If it is one, we spell the numeral with -st ending. As you can see, 25 doesn’t end with number one, but with number five. Therefore, the only correct version is the 25th, not 25st.

Incorrect spelling

Incorrect spelling