Christmas day, or a Christmas carol. A contraction of nouvelles (tidings), written in old English, nowells.
“A child this day is born, A child of high renown, Most worthy of a sceptre, A sceptre and a crown. Nowells, nowells, nowells! Sing all we may, Because that Christ, the King. Was born this blessed day.” Old Carol.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894