YOU SAID:

He did say his first day of

INTO JAPANESE

彼は彼の最初の日を言った

BACK INTO ENGLISH

He said his first day.

INTO JAPANESE

彼は初日を言った。

BACK INTO ENGLISH

He said the first day.

INTO JAPANESE

彼は最初の日に言った。

BACK INTO ENGLISH

He said on the first day.

INTO JAPANESE

彼は初日に言った。

BACK INTO ENGLISH

He said on the first day.