First up, full disclaimer, I am not Welsh, I live here, it is my home by choice, I have no direct Welsh ancestry, and my Welsh language skills are not exactly stellar...
However, one of the parts I love about living in this part of Wales is the number of old traditions that have been kept alive. I live in that funny bit of Wales that's right in the middle, there are no large towns, and even the form of Welsh they speak round here gets described as archaic by certain Welsh language speakers who I know.
The idea of a Christmas Eve service at midnight is one that's pretty common in many parts of the Christian world. In this part of Wales they take it one step further. They hold their service between 3am and 6am, with the night before spent decorating the houses, and meeting up with friends. During the service the church is filled with candles.
This is a description of the service in Dolgellau during the middle of the last century.
"Now the church is in a blaze, now crammed, body, aisles, gallery, now Shon Robert, the club-footed shoemaker, and his wife, descending from the singing seat to the lower and front part of the gallery, strike up alternately, and without artificial aid of pitch pipe, the long, long carol and old favourite describing the Worship of Kings and of the Wise Men, and the Flight into Egypt, and the terrible wickedness of Herod. The crowds are wholly silent and rapt in admiration. Then the good Rector, and his curate, David Pugh, stand up, and read the Morning Service abbreviated, finishing with the prayer for All Conditions of Men, and the benediction restless and somewhat surging is the congregation during prayers the Rector obliged sometimes to stop short in his office and look direct at some part or persons, but no verbal admonishment. Prayers over, the singers begin again more carols, new singers, old carols in solos, duets, trios, choruses, then silence in the audience, broken at appropriate pauses by the suppressed hum, of delight and approval, till between eight and nine, hunger telling on the singers, the Plygain is over and the Bells strike out a round peal."
The carols are sung without musical backing, and there is a certain competitive nature to the singing. Small groups stand, sing a carol, and then another group will stand and present their carol. Within a service there will be several groups taking part, and they will each sing in turn. There is no set programme of songs, and it's a point of pride that no carol is repeated.
Plygain is no longer just a Christmas Eve tradition. Throughout the winter local churches will host their own plygain, and groups from other churches will visit and sing. The Say Something in Welsh site has a list of various Plygain services (though there will be many more happening). There is even one in London on 6th January.
The other mid-winter event that still takes place here is the Mari Lwyd.
This is a New Year tradition, involving a man under a white sheet, holding a horses skull. Traditionally it would go around the houses in the village, and a door-to door challenge in Welsh verse would take place. That's mutated in to a trip around the pubs in our villages...