It hasn't been announced by Microsoft, it is speculation from the waypoint forums. One user claims that he spoke with a customer service rep on the phone and they stated that codes would start going out at midnight gmt. Whether you want to believe this user is up to you.
Link to thread - https://forums.halowaypoint.com/yaf_postst154602_Halo-4-Specializations-Email-Facts.aspx
http://www.youtube.com/watch?v=32p8d6OudgU Xbox Live Community Ambassador GT - MegaAtomBomb