Boxing Day is so called because on this day it was the customary for tradesmen to collect their Christmas boxes or gifts in return for good service throughout the year. Also, it included giving money and other gifts to charitable institutions, and the needy.
The holiday may date from as early as the Middle Ages, but the exact origin is not known. It may have begun with the Lords and Ladies of England, who gave Christmas boxes/gifts to their servants on December 26, or maybe by priests, who opened the church's alms (charity boxes), and distributed the contents to the poor and needy.
