Stores will be able to give copies away from the 23rd of March, wit the movie coming out on the 25th.
Indeed, DC appear to be calling it Dawn Of Justice Day.
So they may have a few other things planned.
Stores can order additional copies to give away, and will get an extra 50 for free if thy order in at least 10 copies (in total) of the following trade paperbacks; Batman: Hush TP, Batman: The Dark Knight Returns 30th Anniversary Edition TP, Batman/Superman Vol. 1: Cross World TP, Injustice: Gods Among Us Vol. 1 TP, JLA Vol. 1 TP, Justice League Vol. 1: Origin TP, Luthor TP, Superman/Batman Vol. 1 TP, Superman/Wonder Woman Vol. 1: Power Couple TP, Wonder Woman Vol. 1: Blood TP