Doing a little Google research reveals if not outright scientific data, then certainly plenty of anecdotal evidence to confirm the spawn does seem to center on the full and new moon. Again though, mainly because all activity seems to peak on those two phases of the moon, which points to the Solunar tables. But it does require the right conditions as well, of course. And as catchNgrease mentioned above, there are several days preceding and following the actual full moon so there's about a week of spawning activity I'd guess. New moon and full moon are approximately two weeks apart, so during spring-summer spawning season they'd be engaged in spawning activity more often than not.