When, exactly, is Twelfth Night?

For those who think that the First Day Of Christmas is Boxing Day,
then it is Jan 6th.

If Christmas Day is the First Day Of Christmas, then it is Jan 5th.

But in Ye Olden Days, the next day started at Sunset and not at
Midnight, so that Christmas Eve was actually part of Christmas Day
and if Christmas Day is the First Day Of Christmas then Twelfth
Night has to be Jan 4th! eg, the Sussex Carol, "On Christmas Night
all Christians sing ... "

Why is this important?

Well, were I to want a Lockdown-Beating party of 100 souls, then
obviously I'd need to be pedantic and at least get the day correct!

:-)