I've actually never heard of this, and had to Google it as well. It seems to be similar to Veteran's Day here in the U.S. (we actually get the day off from school) but we don't do anything like those two minutes of silence. We don't seem to do much at all for the holiday, even though much of us get the day off. Even when we do commemorate events by silence, in the U.S. it's a short moment of silence, usually not even a minute. I just can't see most of the people holding their silence for two minutes to commemorate an event (especially one so old that many, such as me, have little connection to). Still, hats off to the British for this custom.