Well it doesn't celebrate much to tell you the truth. It is a civic holiday up here in Canada. In part it celebrates the Feast of St. Stephen but mostly now it is just accepted as a second day off after Christmas. There are often after Christmas Sales that stores have either on Boxing Day or the day after called Boxing Day Sales as well.
Also my mistake with the date this year. Because Christmas falls on a Friday, Monday will probably be celebrated in lieu of Boxing Day if you don't normally work on the weekends.