Here we come a-wassailing is a traditional Christmas tune but it has almost no mention of the traditional Christmas subjects. The only one I can see in the entire version printed in Wikipedia is in a verse I've never heard before myself:
Bring us out a table
And spread it with a cloth;
Bring us out a mouldy cheese,
And some of your Christmas loaf.
The first verse, though, which is pretty universally known talks about "leaves so green".
Here we come a-wassailing
Among the leaves so green;
Here we come a-wand'ring
So fair to be seen.
Generally one doesn't think of green leaves and winter time. Yes, the chorus says "God bless you and send you a happy New Year", but different cultures have celebrated the new year at different times.
During the Middle Ages in western Europe, while the Julian calendar was still in use, authorities moved New Year's Day variously, depending upon locale, to one of several other days, among them: 1 March, 25 March, Easter, 1 September, and 25 December.
So, was this actually a "Christmas" song, or was it intended to be sung at some other point in the year. If designed for December, what "green leaves" are we talking about?