In the mid-19th century, under Queen Victoria, Boxing Day began as a day on which the economically privileged would bestow gifts on the servile classes. Seasonal gifts to be given to the poor and socially inferior were kept in a "Christmas Box" which was opened on 26th December. In the words of Charles Dickens, Boxing Day was a holiday “on which postmen, errand boys, and servants of various kinds received a Christmas box of contributions from those whom they serve.”
This crumb of seeming generosity from the lucky rich worked on the already-undermined self-esteem of the disenfranchised poor and ensured their continued deference. The ugly social status quo was thus kept intact.
Today, Boxing Day is one on which mixed behaviours are observable, including continuing the seasonal drip-feed of alcohol and general gluttony, going for a bracing walk, and, weirdly, going shopping at megashops like Homebase and Ikea.