Holiday CSS themes - when and what?

Anyone worth half their salt in CSS has used CSS to do site skinning and loves it. The holidays are one of the best opportunities to add a bit of site magic using CSS.

Despite some extensive web searching, I couldn’t answer a more general question. Which holidays do popular sites like to skin around? ... And what dates are best for adding and removing those themes to match users’ expectations? Let’s ask the collective!

a) When do you tend to add and remove your winter holiday theme?
b) What other seasons and holidays do you like to theme around? What are you preferred start / end dates to add / remove those themes?
c) Does anyone have any best practice data from popular b2c sites like Amazon.com and eBay.com?