Originating in the United Kingdom and spreading through the commonwealth countries, Boxing Day is a holiday that extends all the way from the Medieval period when the Lord's serfs were still barred from partaking in the royal Christmas festivities. While Christmas Day is usually spent with family members and often is a very spiritually centered and wholesome time, Boxing Day is the more secularly celebratory day of the Holiday season wherein celebrators gather with their close friends (and perhaps some lingering family members) and get hammered in the spirit of Christmas. So hammered that several drunken boxing fights are known to break out between friends and family members, people whom wouldn't usually be partaking in drunken boxing let alone the act of getting wasted together.
"Man, it's Boxing Day! Let's go out and get schlammered in the spirit of Christmas!"
"Dude... I just celebrated Boxing Day yesterday, I never thought I would get wasted in front of my Grandma..."