Announcements are properly sent within 48 hours AFTER the event happens, whether it is a graduation, a wedding, or a birth.
Be careful sending them out in advance in the hope that recipients will attend the event; most graduations strictly limit the number of admission tickets that each graduate gets. The maximum usually is around 8-10, and depending on the venue, they may limit each graduate to no more than 2 guests. You really wouldn't want to have people plan to come in from out of town and have to tell them that they cannot actually attend the event.