Voracious readers probably read more during vacation times, not less. I'm living proof. Before I retired from teaching, the vast bulk of my reading was done during summer.
(I expected Easter to be a slow time, but sales didn't completely die, and it was pretty good for KU reads (best day so far this month).
Something I noticed from the time I started publishing--not everyone's cycle is the same. I haven't seen that many threads like that recently, but I used to see threads all time time on various forums about why month X was a slow time, If one put them all together, it would seem that every single month was a slow time. And there was always people joining the discussion to say that month X was their best month.
It used to be fairly consistent that December wasn't great for ebooks because you couldn't put them under the tree, but January was good because of all the new kindles. As the kindle market became more saturated, the January bump lessened a bit. But aside from that, I don't remember seeing any consistent trend. Months with new releases in them tended to be better, regardless of the season. And my worst month for advertising effectiveness during this last year was November, for no particular reason. (November last year was decent.)
I'm with writeway on this issue--slow times don't mean that advertising will be less effective. But less advertising may mean lower sales when times would otherwise have been better.