Boxing Day - the Day after Christmas!
Boxing Day takes place on December 26th and is only celebrated in a few countries; mainly ones historically connected to the UK (such as Canada, Australia, South Africa and New Zealand) and in some European countries. In Germany it is known as "Zweiter Feiertag” (which means 'second celebration') and also “Zweiter Weihnachtsfeiertag” which translates as Boxing Day (although it doesn’t literally mean that)!
The dating of Boxing Day can get rather complicated. You might also see it described as 'the first weekday after Christmas'. However, that's describing the 'Public (or Bank) Holiday' for Boxing Day in the UK. I think it's possible to separate Boxing Day 'the day' (the 26th) and 'the bank holiday'. They're normally on the same date but don't have to be. The same is true for Christmas and its bank holiday! In 2021, Christmas Day and Boxing Day (the 25th and 26th!) are on a Saturday and Sunday. So their bank holidays are on the 27th and 28th of December - a Monday and Tuesday. But that doesn't mean that 'Christmas' 2021 will be on Monday the 27th! So if you're going by the bank holiday definition of Boxing Day, then it should be described 'the first weekday *after the Christmas bank holiday*' not *after Christmas* - as they can be different too! On the UK government's site listing the bank holidays in the UK, it states "If a bank holiday is on a weekend, a ‘substitute’ weekday becomes a bank holiday, normally the following Monday."; this make more sense than saying Boxing Day or Christmas Day are 'on' other dates!
The 26th December is also St. Stephen's Day (or 'the feast of Stephen'). Just to confuse things, there are two St. Stephens in history! The first St. Stephen is believed to have been a very early follower of Jesus and he is said to have been the first Christian Martyr (a person who dies for their religious beliefs). The Bible says that Stephen (who was a Jew) was stoned to death by some other Jews (who didn't believe in Jesus).
The second St. Stephen was a Missionary, in Sweden, in the 800s. He loved all animals but particularly horses (perhaps why there is traditionally horse racing on boxing day). He was also a martyr and was killed by pagans in Sweden. In Germany there was a tradition that horses would be ridden around the inside of the church during the St. Stephen's Day service!
Both St. Stephens have been associated with charity and giving for a very long time; and historically that's what St Stephen's Day/Boxing Day was about.
Starting in the Middle Ages, it was the day when the alms box, collection boxes for the poor often kept in churches, were traditionally opened so that the contents could be distributed to poor people. Some churches still open these boxes on Boxing Day.
It might have been the Romans that first brought this type of collecting box to the UK, but they used them to collect money for the betting games which they played during their winter celebrations! In The Netherlands, some collection boxes were made out of a rough pottery called 'earthenware' and were shaped like pigs. Perhaps this is where we get the term 'Piggy Bank'!
It was also the day when rich land owners would give 'gifts' (often some leftover food from the main Christmas feast!) to those who worked and lived on their land; and later on it became traditional that servants got the day off to celebrate Christmas with their families on Boxing Day.
Before World War II, it was common for working people (such as milkmen and butchers) to travel round their delivery places and collect their Christmas box or tip. This tradition has now mostly stopped and any Christmas tips, given to people such as postal workers and newspaper delivery children, are not normally given or collected on Boxing Day.
St Stephen's Day is when the Carol 'Good King Wenceslas' is set. That carol is about a rich King helping the poor. It was written in 1853 and reflects the Victorian view of being charitable at Christmas. You can read about the real history of Wenceslas here...
Boxing Day has now become another public holiday in countries such as the UK, Canada, Australia and New Zealand (If Christmas is at the weekend, normally the next working day is made into a public holiday as a 'substitute' for Boxing Day). It is also the traditional day that Pantomimes started to play.
There are also often sports played on Boxing Day in the UK, especially horse racing and football matches! In Australia there is the cricket Boxing Day Test Match, where Australia play another country. Boxing Day is also when shops traditionally had big sales after Christmas in the UK (like Black Friday in the USA).