Gaudete!

Mediaeval Baebes, "Gaudete". Nobody knows the origin of this Christmas carol, but it was probably composed in the sixteenth century; the first physical version is in the Piae Cantiones, published in 1582 by Jacobus Finno. Given the latter, it is also probably Finnish/Swedish in origin (Finland at that time being part of the Kingdom of Sweden), although this is not absolutely certain -- the Latin songs in that collection are from all over Eastern Europe. (The Piae Cantiones gives us a number of important religious songs in English; probably the most famous Christmas one is John Mason Neale's translation of one of the songs into "Good Christian Men, Rejoice".) The most famous version of "Gaudete" in recent times is that of Steeleye Span, which did very well on the charts in 1973 and is (I think) still the most successful all-Latin song ever to chart in the English-speaking world.