I'd not heard they ran "every Friday," but they usually run four or five times during contests, and from what I've seen over time, randomly once a month or so between contests. I've always thought the timing was random, but then I don't enter contests and only pay idle attention, so don't know for sure. Someone who pays closer attention will drop by, I'm sure.
Odd that this should come up now. I had votes removed from three stories today. There was only one case where it changed the score, and in that case the score went down.