Eve of Twelfth day; evening of January 5 WordNet 3.0 & Open English WordNet

Twelfth Night is a Christian festival on the last night of the Twelve Days of Christmas, marking the coming of the Epiphany. Wikipedia

A Christian festival marking the coming of Epiphany and concluding the Twelve Days of Christmas, traditionally falling on the evening of January 5 (i.e., on the eve of Twelfth Day, January 6), but also sometimes defined as falling on the evening of January 6 (i.e., on the evening of Twelfth Day itself). Wiktionary