Shadowmonkey recently mentioned always having the same pattern to his sales which is quite an achievement in itself, instead of having a downward slide.
I've also noticed some inexplicable patterns. I guess they just happen and we will never know why.
One of the books I give away is consistently downloaded 3 to 4 times more on the iBookstore than it is on Kindle. Of those iBookstore downloads the USA store is pretty consistent each day whereas the UK, Canada, Australia and Ireland downloads fluctuate quite a lot. Generally there are more downloads during the week than there are at weekends.
Sales are much more random, with the Nook book store contributing more than I thought it would and Kindle less than I thought it would.