Luckily I am sharing boxing day with close friends, their family and my mother too and we have a great time and don't even think about boxing day sales. I don't like massive crowds and don't need anything that desperately. Mind you I think there a a few bargains to be had but you need to want them. These days there are constant sales everywhere and all the time to entice us.
I look it up, according to the OED (via Wikipedia): "Various competing theories for the origins of the term boxing day circulate in popular culture, none of which are definitive. However, the Oxford English Dictionary gives the earliest attestations of the term as being from England in the 1830s, defining it as 'the first week-day after Christmas-day, observed as a holiday on which post-men, errand-boys, and servants of various kinds expect to receive a Christmas-box'."