The Hymns and Carols of Christmas Website does a good job of setting the context. Remember that the lyrics for this song were written by John Mason Neale, and published in 1853. The song was based upon Neale's Legend of St. Wenceslaus. The historical Wenceslas was Duke Vaclav of Bohemia.

The melody is Tempus adest floridum ("Spring has now unwrapped her flowers") from Piae Cantiones (1582) - often known as the Flower Carol. Piae Cantiones is also the source of "Gaudete."

-Joe-