Boxing Day is an English bank (public) holiday.
Currently a favourite for people wanting to grab a bargain in the post Christmas sales and for sporting events including, one day cricket and soccer. The day is not known for boxing.
A round robin of sites courtesy of Google yielded the following theories for the origin of Boxing Day:
Alms boxes
In the Middle Ages (approximately late 5th – late 15th centuries), it was common for churches to open alms boxes on the day after Christmas, distributing all the money inside amongst the poor.
Christmas boxes #1
In Queen Victoria’s reign (1819 to 1891) servants of wealthy household were given a rare day off to visit loved ones. Masters would gift a Christmas box, containing food, gifts and sometimes a financial bonus.
Christmas boxes #2
Tradespeople such as milkmen and butchers would spend the days after Christmas collecting money or gifts left to them by customers. As a child, I remember my mother putting aside some Christmas box money to give to the men who delivered bread, pop (soft drink), milk etc.
After the excesses of Christmas Day, we are spending Boxing Day relaxing with our Sydneysider friends. This evening we will join dear friends for a rooftop barbecue in Newstead, Brisbane.
theINFP 
Here in the States “Boxing Day’ is coming into fashion for sales and greetings although no one here has the slight inkling what Boxing Day is about.
Your Boxing Day sounded very chilled and relaxed – love the thought of the barbecue! Interesting how Boxing Day was interpreted, so to speak, in earlier times, we’re much more self-centered these days, which is a real shame. Those baubles in that image are awesome! Wouldn’t mind some for myself 😀 I’m sure my Christmas tree would thank me!
The baubles are around twenty years old. I’m sure you could find some on Etsy ❤️
I could certainly give it a go!