Plygain is a traditional Welsh Christmas service which takes place in a church between three and six o'clock in the morning, traditionally on Christmas morning.
The word is first recorded in the Black Book of Carmarthen in early Welsh manuscripts in the 13th century ("pader na pilgeint na gosber"). Many Welsh Christmas carols are traditional, and could be called plygain in some circumstances, their tone being quite different to the Christmas carols that we know today, with many of the old Plygain songs in the Dorian mode.
Some of the Plygain carols are thought to have been created to replace the traditional Latin Catholic mass after the 1588 Welsh translation of the Bible; Plygain carols were a mainstream stay of Welsh protestant worship until the mid-19th century. Peaking in popularity in the 17th century, author Charles Edwards (c. 1628 – c. 1691) published a book of carols called Llyfr Plygain gydag Almanac (Plygain Book with an Almanac) in 1682. The decline of the tradition declined in the mid-19th century was attributed to the rise of family gathering as an alternative Christmas-eve tradition and a 'Victorian' rebuff of the joyous celebrations which went with the Plygain. Some parishes had to abandon the practice after repeated incidences of drunk villagers disrupting the services.
Most carols were written down including the works of plygain composer by Huw Morys (1622–1709) and many families who had their own songs, passed down from generation to generation. Many of these are still sung today. In the 18th century a number of plygain were written by Jonathan Huws in his book Bardd y Byrddau, including Carol Plygain on the music of Gwêl yr Adeilad. Another key author in the 19th century was intellectual Gwallter Mechain. The poet Thomas Williams (c. 1769–1848), from the Llanfyllin area and Llanfihangel-yng-Ngwynfa, who published several volumes of carols and plygain. In the 20th century, Canon Geraint Vaughan-Jones, a clergyman dedicated to preserving this tradition, published a collection of Plygain carols under the title Cyff Mawddwy, and this was republished by Lolfa Press in 1987 as Hen Garolau Plygain.