You should be able to drink at midnight on the day before you are 21 because by law you are 21 on the day before your 21st birthday. The only exception would be if the state has a statute that specifically states that you are not allowed to drink until your "21st birthday" as opposed to being "21 years of age."

There are two legal maxims at work here. One is "A day begun is a day done." The other, very similar, is "The law does not recognize a part of a day."

If a person is born on January 1 he is in existence for the whole day even if he was born at 11:59:59 p.m. Then as soon as soon December 31 begins, at midnight between the 30th and 31st, he has been in existence for 365 days. One year. So he is one year old on the day before his birthday.

A person does not need to be in existence for 365 24 hour periods to be a year old, otherwise we would all have our time of birth as well as out date of birth on our licenses. That's the point about the law not recognizing parts of a day.

There are cases in most states to this effect: Nelson v. Sandkamp, 227 Minnesota Reporter 177; Commonwealth v. Howe, 35 Pennsylvania Superior Court 554; Lenhart v. State, 33 Texas Criminal Reporter 504; State in interest of F. W., 130 NJ Superior Court Reports 512. These are not drinking age cases but the principle still applies.

The Federal view as to turning of age is the same. See Turnbull v. Bonkowski, 419 F. 2d 104. This is how the court explained it:

"Since one is in existence on the day of his birth, he is, in fact, on the first anniversary of his birth, of the age of one year plus a day or some part of a day. The appellant did, then, reach the age of nineteen years on the day before the nineteenth anniversary of his birth,. . . '

At common law the rule was the same. This is what that legal scholar Blackstone said

"Full age in male or female is 21 years, which is completed on the day preceding the 21st anniversary of a person's birth." 1 Blackstone's Commentary 457.

This is how lawyers and judges have viewed the issue, but you might not be able to convince a bartender of the accuracy of this answer. I hesitate to advise printing this answer and giving it to a bartender (even with all the WikiAnswers adornments). The bartender might just think you've already been drinking and escort you to the door.

So you can have a drink at midnight on the day before your 21st birthday, but if you do, drink responsibly. And don't drive afterward.