The entry in my copy of Brewer's Dictionary of Phrase and Fable gives the following information.

Small gratuities given on Boxing Day. Boxes placed in churches for casual offerings used to be opened on Christmas Day and the contents called the "dole of the Christmas box" or the "box money" were distributed the next day by the priests. I assume to the needy.

I avoid Boxing Day sales and only watch the cricket highlights. I do enjoy watching the beginning of the Sydney to Hobart yacht race on Boxing Day.

No, it isn't the day Mohammed Ali fought Joe Frasier or George Foreman.  

1 hour ago, Bill W said:

Boxing Day is an an official bank holiday in Britain, Canada, New Zealand and most of Australia.

A bit of clarification about Australia.

In Australia the concept "bank holiday" is very limited. It refers to a holiday for banks and a few other groups. Generally very few Australians would know the date of bank holiday. It is not an official public holiday.

The day after Christmas day is a national public holiday across all Australian states and territories. It is called Boxing Day in all states and territories except South Australia where the same day is called "Proclamation day". Same day, same holiday, different name.