These dates are usually announced by the Joint Admissions and Matriculation Board (JAMB) and the various institutions after the Unified Tertiary Matriculation Examination (UTME) has taken place.
Keep in mind that dates may vary for different institutions and may change year to year. It's always best to check the official website of JAMB and your preferred institution or educational portals to obtain the most accurate and up-to-date information regarding the post-UME examinations.