I see the M & L liturgy planner suggests Messiaen's O Magnum Mysterium for Christmas Day.
Did he write one then?
By way of compensation, I'd warmly recommend the setting by Morten Lauridsen - you need a biggish choir because it's in lots of parts (nine, if memory serves), but it's actually pretty easy, once the choir have got their heads round Lauridsen's simple harmonic vocabulary. And it works well with the organ doubling the voice parts, if a cappella is too much of a challenge. A lovely piece, anyway; listen to it here.
It's a fair cop, guv. My apologies: I meant O Sacrum Convivium. Mind you, that piece has tripped me up before too, when I suggested it for Holy Thursday one year, alleluias and all! It was mcb - bless him! - who rumbled me then.
Some lovely person has posted piles of polyphony on You Tube with a moving landscape score so you can sing along. For example, Byrd's Ave Verum:http://uk.youtube.com/watch?v=KBYLNbKoP ... re=related (Still can't work out how to do links on here!)
Wonderful experience: recording in different key from score. This is good practice for me, whose choirmaster keeps throwing scores at us written in funny clefs - down a 7th, down a third, whatever; or he may decide to transpose a piece on a whim. I'm beginning to get used to it ..... at least the choir stays in tune.
Alas, the piece mentioned above are concert pieces in my experience, not heard in the liturgical circles I move in.