I know from past posts that Gtrsrgr8 lives in the southern US. As a fellow Southerner I can say that although I am aware of Boxing Day it is not a common term around here. The first time I heard of it I mistakenly thought it had something to do with the sport of boxing
In the US, the 26th is typically the day people go to stores to return all the stuff they got for Christmas but didn't want/wrong size/etc.