From SA Mafia Wiki
A Deadline is when the game day is scheduled to end. Typically a day can be ended early if a hammer falls before the deadline. Likewise a day ends on with no executions if the deadline is reached and nobody has the necessary votes to be executed.
Deadlines can often be negotiated with the moderator; if you will be out of town that day, for instance, the mod can push it forward or backward. Sometimes deadlines can be "floating" - they will happen without an exact time or date, occasionally by surprise. Sometimes deadlines are pegged to postings - if no player has posted in 24 hours, for instance, the day might end. Sometimes individual days may or may not have deadlines, but instead the game will have a deadline, where if the game is not resolved by a particular date, the game ends.
Deadlines are universally considered the single easiest and most effective way for moderators to encourage game participating, and ensure their game ends in a timely fashion. Deadlines of about three days are the most reasonable; any more can be too long, and any less too short.