That's not the easiest thing to track. The most obvious trend is the the flurry of posting that occurs around major Apple events. So January (MWSF) and June (WWDC) are big times for us, in addition to some of the special events we've seen in the past.
Things are generally quietest in December around the holidays, and there seems to be a lull in the spring.
It's tough to figure out exactly what's going on because you have the influence of Apple events overlaid on seasonal trends, which are then overlaid on a general upward trend in posting rates as we add users. But in general, fall seems to be busier than spring.
Looking at 2007, we had a peak around MWSF, then things dropped off considerably to levels even below those of the previous fall. We peaked again in June around WWDC and then tailed off a bit but remained high as we then built to another peak for the Leopard release in late October, followed by another decrease.