Incorrect spelling, explanation: this form is incorrect because th is not used as an ending for ordinal numbers with first, second or third at the end. Because 63rd ends with third, we cannot use form 63th. 63rd is the only correct way to write this ordinal number.

Correct spelling, explanation: 63rd is the correct form. There are several suffixes for ordinal numbers: th (the most common), st (for number first), nd (for second), and rd (for third). Since 63 ends with three, thus with third in its ordinal form, rd is the suffix we are going to use – 63rd. 63th is a mistaken form.

Definition of 63rd:
numeral, the ordinal number between 62nd and 64th
It is the 63rd day of Anna’s sobriety. We keep our fingers crossed for her!
On dad’s 63rd birthday we went to Rome.

Incorrect spelling

Incorrect spelling