A Celtic Heartbeat Christmas is a little-known folk compilation but it has some great tracks and is great to listen to while the snow is falling. It's been in my family since the late 90's.
Other Christmas time favourites are Frank Sinatra, Elle Fitzgerald and Andy Williams to name a few. Yo-Yo Ma's Songs of Joy and Peace album is also very good, and I offer a special nod to The Wexford Carol, featuring Alison Krauss on that album.
