I play games while school is in session because it is a lot easier to stop and play a game between doing stuff for school as opposed to reading a little bit and still getting something out of it.
 
For me, personally, I like to read a lot if I am going to be reading.  I got a lot of really interesting books for Xmas and I am excited about hunkering down with them; which is what I have been doing.  I got a few games as well, and I am really excited about playing them - and have - but I know that they will get more of my time once school is back in on January 11th.
 
The other reasons I prefer to read for my own pleasure during the breaks are 1.)  Relaxing on the couch and just reading a good, interesting book feels like a vacation to me and 2.) I have to read so much when school is in session that I abhor the idea of reading on my own time because of it.