Time will tell, however I'm skeptical about the symbolic anniversaries theory. Almost any date is an anniversary of something "interesting," and although some of the events have been on dates with some mild degree of "interest," none of them have been on dates that would be really obvious: for example, the battle of Tours was in October.
More importantly, August 6th is a Saturday this year. For optimal loss of life, such an attack should be planned for a weekday, preferably a Tuesday, Wednesday, or Thursday, when the most people would be at work in the central parts of the cities. In fact many of the attacks have in fact been in the middle of the week. My guess is that that won't be the date of the next big attack though I admit that it's not much more than a hunch.
History by the numbers...
History by the moon charts...
History by the most holy days...
Jihad, another day.