Ubend, personally speaking (and am a big fan of classical music) I wouldn't recommend the full Messiah to the non-initiated - it has a couple of really nice parts wedged in between a lot of repetitive and not terribly interesting sections - far better to choose something like the christmas carol singalong (the orchestra will definitely play the best bits of the messiah in this) on the 21st or better still The Glory of Christmas on the 22nd (great selection of christmas classics)
just my 2p worth though