I seem to recall that Amazon uses an X day rolling average on the app store, and I would imagine something similar with books. My guess is more experienced kdp authors have a pretty good sense of how long it takes the freebies to maximize rank, and try to get that effect into the system in advance of high traffic days to actually sell well. I didn't really see my title rise in rank much until after cyber monday, and I still don't have a great sense of how long it takes for a spike in downloads to boost the rank to peak. 2-5 days? If my pre-thanksgiving and pre-cybermonday downloads were high, perhaps it's because those in the know scheduled their promo days earlier so they'd peak in rank on the busier days. Had I scheduled a little earlier, I might have been dwarfed then too.
@Vydor, I plan to only use one more free promo this year, and save the last for next. I want to target the pre-travel periods, since I think my ebook is attractive for short and low commitment bursts of travel reading. After Christmas might be a good time indeed, both since there are new devices, trips home, and perhaps because the other books might (?) focus their freebies pre-Christmas to drive up their ranks to make actual sales during that time, leaving less competition among freebies.
But knowing there are better and worse days to promote out there, I'd hate to waste another on a day with too much competition, and it's all guesswork right now for me.
@kenny,
if it makes you feel any better, I don't have much to show in the actual purchases category and I'm sure you've bested me there many times over.