According to Sir James Frazer, author of the justly celebrated The Golden Bough, the day after Christmas in ancient Britain was solemnized with a very odd ritual. Although killing or harming a wren was considered highly unlucky most of the time, it was customary among certain Celtic peoples to hunt and kill a wren on Christmas Day or the day after (the feastday of St. Stephen). Frazer relates the ritual as it was carried out on the isle of Man:

"On the twenty-fourth of December, towards evening, all the servants got a holiday; they did not go to bed all night, but rambled about till the bells rang in all the churches at midnight. When prayers were over, they went to hunt the wren, and having found one of these birds, they killed it and fastened it to the top of a long pole with the wings extended. Thus they carried it in procession to every house chanting the following rhyme:

"We hunted the wren for Robin the Bobbin,
We hunted the wren for Jack of the Can,
We hunted the wren for Robin the Bobbin,
We hunted the wren for every one."

When they had gone from house to house and collected all the money they could, they laid the wren on a bier and carried it in procession to the parish churchyard, where they made a grave and buried it 'with the utmost solemnity, singing dirges over her in the Manks language, which they call her knell; after which Christmas begins.' The burial over, the company outside the churchyard formed a circle and danced to music."

A similar custom prevailed in parts of Ireland, although the rhyme is a bit more to the point:

"The wren, the wren, the king of all birds,
St. Stephen's Day was caught in the furze;
Although he is little, his family's great,
I pray you, good landlady, give us a treat."

In Irish tradition, the treat was usually money or food.

If any of this sounds similar to Boxing Day, it's probably not a coincidence.