Wikipedia has a good article on the song. I've never thought of it as a Christmas song, but I guess it could fit for Christmas. I've always thought of it as a Eucharistic hymn, and Wikipedia says it goes back to the Divine Liturgy of St. James in 275 AD or earlier. The post from Dzechman above is a pretty good summary of the Wikipedia article.
Hymnary.org refers to the song as "a lesser Christmas hymn." In my Roman Catholic tradition, it was sung as a Eucharistic hymn, not particularly a hymn for Christmas.

-Joe-