Childermas

With much feasting they fared the first and the next day,
and as heartily the third came hastening after:
the gaiety of Saint John's day was glorious to hear;
[with cheer of the choicest Childermas followed,]
and that finished their revels, as folk there intended,
for there were guests who must go in the grey morning.

As a boy in his early teens, St John the Beloved Disciple was of course the youngest of the Apostles. 'Now', says Guéranger, 'just as St Stephen is looked upon as the type of Martyrs, St John is honoured as the Prince of Virgins.'

Martyrdom won for Stephen the Crown and palm; Virginity merited for John most singular prerogatives, which, while they show how dear to God is holy Chastity, put this Disciple among those who by their dignity and influence are above the rest of men.

[Guéranger]

The line in square brackets in the first, verse quotation above is in fact Tolkien's own "reconstruction": he notes that his author seems to have missed out 28th December from Sir Gawain's busy schedule, but it is possible that he has misconstrued the mood of the day. There are in fact two traditional characteristics associated with the feast commemorating the Church's earliest and youngest martyrs - specifically tragedy and (as Roud puts it) 'a particular regard for children'. Fasting and penance on the part of adults would go hand-in-hand with a certain degree of licence for young ones, Boy Bishops, and so on.

[The Church] sanctions the practice, observed in Cathedrals and Collegiate Churches, of allowing young boys to share in the duties of the Choir, and blend their innocent chanting with that of the Ministers of God. She grants them several privileges, and takes pleasure in seeing the delight wherewith these children perform the several functions entrusted to them. This joy, this simplicity, this innocence, all add a charm to the divine Service; and through these youthful Choristers the Church pays honour to the Infant Jesus, and to the Holy Innocents of Bethlehem.

[Ibid.]