We have a picture storybook "The Night before Christmas" which is all about Santa's busy night. It starts with the verse:-

Twas the night before Christmas, when all through the house
Not a creature was stirring, not even a mouse;
The stockings were hung by the chimney with care,
In hope that Santa Claus soon would be there.

Christmas traditions like "deck the halls with boughs of holly" go back many centuries - using words like "Tis" and "Twas" is perhaps an attempt to retain the authenticity of the ancient stories and rituals.
We have four young grandchildren - twill soon be time for another generation to know that at Christmas "Tis the season to be Jolly", though tra la la, we don't have a harp!

While outlining his plans to take England out of lockdown on 2nd December, Boris Johnson said;-

"Tis the season to be jolly, but it is also the season to be jolly careful".

I think he was talking about Covid, but since he will not say how many children he has fathered (at least six, including one where he denied paternity for four years), I cannot be sure about this.