In different parts of the world, the First Day of Christmas, December 26, is known as St. Stephen's Day, the Day of the Wren, or Boxing Day.
December 26th is a day devoted to remembrance of the Christian martyr, St. Stephen. In countries where St. Stephen is still celebrated, people devote the day to spending time with friends and family.
St. Stephen is believed by many to be the first Christian martyr - he was stoned to death sometime around 33 CE.
St. Stephen's Day has been a holiday in Ireland for hundreds of years, where it is known as The Day of the Wren, and is still a public holiday today. The wren is related to St. Stephan because of stories that a wren betrayed St. Stephen's presence while he was hiding from his enemies.
As was discussed in this blog post, in the Middle Ages the First Day of Christmas was a day when earthenware boxes full of coins were distributed to servants. Churches also circulated donation boxes amonts the congregation during Advent, and those boxes and their contents were distributed amongst the poor on the day after Christmas.
Several countries still celebrate Boxing Day - including Scotland, Ireland, Australia and Canada. In some Canadian provinces Boxing Day is a statutory holiday when all workers are given a mandatory day off with full pay. In the countries that still celebrate Boxing Day, it is much like the day after Thanksgiving in the United States - a huge shopping day. Which is really kind of ironic given the historical bases for the holiday.
Beggars Receiving Alms at the Door of a House, 1658, by Rembrandt Van Rijn