The way I observed G*'s tweaking over the past few years at this time is, for holiday SERPs, they push slightly harder products / gifts led sites (traditionally sought after consumer goods at this time of the year ) predominately in countries with Latin based / derived first languages (English, French, Spanish, Italian...). They do the same for other countries respective cultures as well (Eid El Fitr/Adha, Hanouka,...), it does make sense, most people are looking for those consumer goods at this time and sites offering them will get a leg up depending on their trust rank and authority etc. For Normal SERPs, short answer is the off peak season, where the search is even in almost all goods, services and information (unless a disaster...). This is Interesting come to think of it, Hanouka is Jewish, I wonder if people in Palestine / Israel do the same in terms of buying a lot of gifts, if they do and as I understand it, Hanouka is in December, and this year, Eid El Adha in November, would G* presents the same SERPs and algo?
Children's toys and Christmas related gift goods for example are at their peak demand for November / December outranking all sought after goods in the western world and G* does a big shakeup at this time, many sites find themselves ranking well and other loose ranking, in most cases, the situation reverts around about February / March. That's what I seen happen since 2003 anyway.