"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".
Oxford English Dictionary
It is only with our modern understanding of astronomy and time keeping that we start a new day in the middle of the night. To our ancient ancestors, the end of day was when the sun went down. The oncoming night was the beginning of the next day. Nights were actually part of the holiday i.e. part of the next day. visit projectbritain.com
Christmas, started at sunset on 24 December. They considered this to be Christmas Evening (or Christmas Eve). Christmas would then continue until nightfall on the 25th which started St. Stephen's Day, (Boxing Day) the first of the twelve days of Christmas. v
Click here to find out more about the Twelve days of Christmas
To our ancient ancestors, the Twelfth night after Christmas would have been the evening before 6 January, the twelfth and final day of the Christmas season.
The actual date for Twelfth Night has caused much confusion for years and still does today. This is because our modern understanding of the words 'eve' and 'night' mean something different to what our ancient ancestors considered them to be.
For us today the night or evening follows the day:
To our ancient ancestors, it was the other way round:
Twelfth Night Traditions
Why is it unlucky to take down decorations after Twelfth Night?
When is Twelfth Night? | Twelfth Day
"What a good, clear explanation of something which has always been a bit of a puzzle ie. which actually is Twelfth Night and when should decorations be taken down - on or before Twelfth Night." Jakijac
"Thank you for the very clear explanation of twelfth night which has finally settled a very long running argument in the office about twelfth night being the 5th or 6th. Thanks You!" Brian Sheldon