Some bars operate on a "business day" model and may not recognize your new age until they reopen for a new shift later that day.
In many jurisdictions, driver’s licenses expire on your 21st birthday. An expired ID is not a valid form of identification for purchasing alcohol. can you buy alcohol on your 21st birthday
In most states, the "Cinderella rule" applies: at 12:00 AM on your birthday, you are legally of age to purchase and consume alcohol. Some bars operate on a "business day" model