Harry, there are actually three semesters in a year, but most people do not do summer semester. There are essentially holidays (rather, more like unscheduled periods) between all the terms: Christmas (winter), Spring Break, and Summer Vacation (or, if you do summer classes, a two-week break in like August before autumn). Also, if I understand correctly, I believe we have lots of schools on a similar system to your trimester thingy. We call it "quarters", because there are four, one for each season, and one (our summer) is scheduled with the understanding that most everyone will be on extended holiday. Fall Quarter starts in early to mid-September and goes until December when you have a break for Christmas, Winter Quarter is from january until Spring Break (in late March or early April), Spring Quarter is from April to the end of June when there is a break for Independence Day/summer stuff, and then Summer Quarter that no one really pays any attention to that lasts from whenever to some other time around the beginning of September, when we have a bank holiday called Labor Day.