Traditionally it's a day when people ' let their hair down', doing what they love best - sport fixtures like premier football and horse- racing are extremely popular. Upper- class people often go fox- hunting, in their ' pink' riding coats ( red too anybody who ain't colour- blind, lol ), with packs of specially- bred hounds.
Parties, get- togethers, drinking sessions, theatre, pantomimes, long walks in the country, pool games, etc etc , whatever people enjoy.
In the old days when they all went to church on Christmas Day, having a massive dinner afterwards, followed by a nap and a sing- song, people looked forward to the carefree atmosphere of Boxing Day.
You might already know that its name originated because trademen like the milkman, newsagent, etc used to call on that day for their seasonal ' tips' or Christmas Boxes....