The attack was a revenge attack for the drone killing of Al Libi, the Lion Of Libya, in June.
Al Qaeda, Zawahiri had never acknowledged the death of Al Libi in June and the US never claimed to have killed Al Libi because they didn't know for certain that the drone attack on Al Libi had been successful.
It was not until 9-11-12, the day before, that Zawahiri released the tape in which he acknowledged that Al Libi had been killed and called for revenge.
Zawahri may have released the tape the day before, but I’m not so sure that this was all planned in a day.
If it was, & he was just doing it on Sept 11 because of the date, then it would seem that Stevens being there was a coincidence.