K.14.1- Cards of type "Instant" are played when you can play an instants, and they are put into the graveyard when they resolve. [CompRules 1999/04/23]
K.14.2- An Instant cannot come into play. If one would come into play, it remains where it is instead. [WotC Rules Team 2001/08/01]
K.14.Ruling.1- Abilities of permanents commonly are played whenever an instant can be played, but they do not cause the permanent to go to the graveyard. [D'Angelo 1999/02/03]
Note- Instants can be used at many times during both players turns. See Rule T.1.4 and Rule T.1.5.