Which Christmas carol has a tune, "Tempus Adest Floridum", originally praising the springtime?
Although we now tend to associate the word "carol" only with Christmas, traditionally carols were also sung to celebrate Easter/Springtime, too. This tune, dating back to the 13th century, but not formally published until the 16th, in Finland, had Latin lyrics that rejoiced in the coming of the time of flowering.
The words we are most familiar with today were not written until 1853, by the English hymnwriter John Mason Neale (1818-1866).
There was, indeed, a real Wenceslas (or Vaclav, in Czech) in Bohemia in the 10th century, though in his lifetime he was only a Duke. His family were only very recent converts to Christianity - his maternal grandfather was a pagan chief. But he seems to have been a pious man who was generous to the poor, though there is no evidence of the particular incident recounted in the carol of his touching trip to the poor man's house, accompanied by his faithful page. Although Neale set his story on the "Feast of Stephen" - i.e. Boxing Day, the 26th of December, Wenceslas' own Saint's Day is the 28th of September.
He met a rather grisly end, not mentioned in the carol for obvious reasons, run through by his own brother after being set on (in an uncanny fore-echo of Thomas Becket's fate!) by three noblemen.
Since Neale's version was first published, many, including notably Ralph Vaughan Williams, have poured scorn on "Good King Wenceslas" but it seems likely to remain one of our Christmas staples.