Twelfth Night

Today is January 5, the eve of Epiphany. Tonight would be Twelfth Night if you count Christmas as the first night.

From Wikipedia
In some traditions it is taken to mean the evening of the Twelfth Day itself, the sixth of January. This apparent difference has arisen probably due to the old custom of treating sunset as the beginning of the following day. Therefore Twelfth Night would have been considered as occurring on the twelfth day, though in present day custom it is the previous day.