its like grammar, what is strictly "wrong" by academic views/records can actually become correct by common usage.
I realise the OED def and the bank holiday history etc.
Just shows how argumentative and pedantic this site can be!
Can imagine your conversation when a colleague says "oh Saints are home to Exeter on Boxing Day"
"No," Trousers replies. "Actually its not Boxing Day, you need to look at the Oxford Dictionary definition, Boxing Day is actually the holiday for the first working day after Christmas, you are thinking of..."
Trousers looks around and realises his colleague has disappeared.