Down in yon Forest
Wassail track 9
The 25th Track 11
Learned from The Oxford Book of Carols. Vaughan Williams collected the song in 1908 from Mr J. Hall of Castleton in the Derbyshire Peak District. This was the first time the song had been collected from the oral tradition, but the carol itself dates back to the sixteenth century, when it was publsihed by a London grocer, Richard Hill.
You'll find many theories about the mystical significance pf the lyrics. Malcom Douglas had this to say at Folkinfo.org:
Anne Gilchrist quoted the 16th century text along with that from Notes and Queries in the Journal, together with a very detailed discussion of the imagery, linking the song with the Troubadour tradition and suggesting connections with the Grail myth and Mithraic tradition. This was backed up by G. R. S. Mead. This tentative analysis has tended, subsequently, to be assumed as received wisdom; but should probably be treated with great caution.
On Wassail this is sung by Joanne Acty, with Pete Acty (guitar), Tom Bower (harmonium) and Di Whitehead (cello).
On The 25th, it is sung by Sophie Thurman with Jon Fletcher (guitar, vocals), Andy Turner and Ian Giles (vocals). Preceded by On A Cold Winters Day.