Over the years, the origins of many traditions become corrupted and we can then never be sure of the true reasons. Judy's explanation is the one I tend to believe. But the most popular explanation I've heard over the years was that the 26th December was traditionally the BIG day in the calendar for prize fighting (Boxing Matches). Not strictly the same in modern Britain, but many football fans try to keep the tradition alive

This year is the first that I can remember that the 26th Dec. falling on a Sunday has been called Boxing Day. In the past, it was always moved to the 27th December, as in my explanation above. Professional sport was not allowed on the Sabbath. I have 2 identical diaries on my desk from the same publisher. 2004 lists Sunday 26/12/04 as St Stephens Day only and nothing on 27th. The 2005 version shows Monday 26/12/2005 as Boxing Day
and St Stephens Day.
Is it true that in Ireland, they just call it St Stephen's Day?
Safe and happy cleaning

Ken