it's a day off work
The more common stories include:
* It was the day when people would give a present or boxing box to those who had worked for them throughout the year.
* In feudal times, new year season was a reason for a gathering of extended families. All the serfs would gather their families in the manor of their lord, which made it easier for the lord of the estate to hand out annual stipends to the serfs. After all the Christmas parties on 26 December, the lord of the estate would give practical goods such as cloth, grains, and tools to the serfs who lived on his land. Each family would get a box full of such goods the day after Christmas. Under this explanation, there was nothing voluntary about this transaction; the lord of the manor was obliged to supply these goods. Because of the boxes being given out, the day was called Boxing Day.
* In England many years ago, it was common practice for the servants to carry boxes to their employers when they arrived for their day's work on the day after Christmas. Their employers would then put coins in the boxes as special end-of-year gifts. This can be compared with the modern day concept of Christmas bonuses. The servants carried boxes for the coins, hence the name Boxing Day.
* In churches, it was traditional to open the church's donation box on Christmas Day, and the money in the donation box was to be distributed to the poorer or lower class citizens on the next day. In this case, the "box" in "Boxing Day" comes from that lockbox in which the donations were left.
pick one