Will the decision come out at midnight?

If a college decision is available on March 29th for example, does that mean that it will it be available by 12am, or sometime later in the day?

May be, may be not. Each school is different.

Usually they do it after school hours .

For many that means they start issuing decisions sometime in the latter part of the afternoon. Some start earlier. Also, often, not every decison is released at the same time, but they instead come out in waves over a period of a few hours. Though there may be some, I am not aware of any that start right after midnight.

Even if midnight was the hour for announcing admission decisions, it would depend on the time zone. Go to bed. Watched clocks never…