It falls on the day after Christmas, when servants and tradesmen would receive gifts, known as a “Christmas box”, from their bosses or employers. It seems to have started around the 1830s.
In Britain, it was a custom for tradespeople to collect “Christmas boxes” of money or presents on the first weekday after Christmas as thanks for good service throughout the year. This custom is linked to an older English tradition: since they would have to wait on their masters on Christmas Day, the servants of the wealthy were allowed the next day to visit their families. The employers would give each servant a box to take home containing gifts, bonuses and sometimes leftover food.
The European tradition, which has long included giving money and other gifts to those who were needy and in service positions, has been dated to the Middle Ages. It is believed to be in reference to the Alms Box placed in areas of worship to collect donations to the poor. Also, it may come from a custom in the late Roman/early Christian era, wherein metal boxes placed outside churches were used to collect special offerings tied to the Feast of Saint Stephen, which in the Western Church falls on the same day as Boxing Day.