Boxing Day, 26 December, takes its name from the Christmas box. According to the Phrase Finder, the name comes from “the giving of ‘Christmas box’ gifts to tradespeople.” These were gifts, usually money, for people “who have rendered some service throughout the year,” such as milkmen.
This meaning of Christmas box dates back to at least the early seventeenth century. The earliest reference to the name Boxing Day that the Phrase Finder found dates from 1805.