I have long been confused by conflicting references to Twelfth Night as either the evening of the 5th or of the 6th of January. Wikipedia’s take (link):
It is defined by the Shorter Oxford English Dictionary as "the evening of the fifth of January, preceding Twelfth Day, the eve of the Epiphany, formerly the last day of the Christmas festivities and observed as a time of merrymaking". However, there is currently some confusion as to which night is Twelfth Night: some count the night of Epiphany itself (sixth of January) to be Twelfth Night. One source of this confusion is the Medieval custom of starting each new day at sunset, so that Twelfth Night precedes Twelfth Day.So tonight it is – the Twelfth Night, with then the Feast of the Epiphany tomorrow closing out Christmastide.
R Balsamo