This is a selection of songs from Piae Cantiones (a book of songs from a school in Finland, compiled in the 1500s).
Other than voices (all women), this disc is completely without instruments, and many of the songs are in unison. Other recordings of Piae Cantiones I have listened to have included fairly complex instrumental parts. This one is my favourite because of its simplicity. It is very beautiful and calming to listen to. (It has a kind of Gregorian-chant aura to it.) Even though not all selections are Christmas songs as such, I find it makes for perfect meditative Advent-listening. Some of the harmonising voices are quite unusual - one of the songs (I have momentarily forgotten its name) includes parallel major seconds, minor seconds in unusual places, and other modern-ish harmonies.
Most of the songs are in Latin and one or two have sections in Swedish.
The booklet that comes with the CD is very good and includes translations of all the selections.