When does a down officially start in football?

A down officially starts in football with a legal snap or a kick on a free kick. This means that once the ball is legally snapped to the quarterback or kicked, the play is live, and all players are allowed to participate in the play. The snap initiates the action, marking the beginning of the down as the ball is put in play.

Other elements mentioned, such as the whistle blowing or the quarterback calling the play, are important processes in the game, but they do not mark the official start of a down. The whistle is used to signal various events, including the end of a play or an infraction; the quarterback calling the play is part of the pre-snap routine but does not mark the beginning of the down. Additionally, the defense being set does not influence when a down starts; it is the snap or kick that officially activates the play.