Wiki:
Boxing Day was traditionally a day on which the servants had a day off from their duties. Because of this the gentry would eat cold cuts and have a buffet-style feast prepared by the servants in advance. In modern times many families will still follow this tradition by eating a family-style buffet lunch, with cold cuts rather than a fully-cooked meal. It is a time for family, parlour games and sports in the UK.
The traditional recorded celebration of Boxing Day has long included giving money and other gifts to those who were needy and in service positions. The European tradition has been dated to the Middle Ages, but the exact origin is unknown and there are some claims that it goes back to the late Roman/early Christian era; metal boxes were placed outside churches used to collect special offerings tied to the Feast of Saint Stephen.In the United Kingdom it certainly became a custom of the nineteenth century Victorians for tradesmen to collect their "Christmas boxes" or gifts in return for good and reliable service throughout the year on the day after Christmas. However, the exact etymology of the term "Boxing" is unclear, with several competing theories, none of which is definitively true.
Another possibility is that the name derives from an old English tradition: in exchange for ensuring that wealthy landowners' Christmases ran smoothly, their servants were allowed to take the 26th off to visit their families. The employers gave each servant a box containing gifts and bonuses (and sometimes leftover food). In addition, around the 1800s, churches opened their alms boxes (boxes where people place monetary donations) and distributed the contents to the poor.
The establishment of Boxing Day as a defined public holiday under the legislation that created the UK's Bank Holidays started the separation of 'Boxing Day' from the 'Feast of St Stephen', and today it is almost entirely a secular holiday with a tradition of shopping and post-Christmas sales starting.