Boxing Day, or 26th December, was the day traditionally when servants of wealthy families would receive a gift, or Christmas 'box' (a box containing a gift, a bonus and food) from their employer and a day off to celebrate Christmas with their own families. Nowadays, it's a day traditionally spent with families, relaxing, playing games, or going for a long walk (thought to help you walk off the overindulgences from the day before).
More recently, Boxing Day has also become synonymous with sport, particularly football and rugby, with many matches taking place. It's also a time for British eccentricity to be seen, as freezing cold swims, fun runs and charity events take place. It's also when the 'Boxing Day' sales start and when bargains galore can be found.