Which is correct?
_______ the night before Christmas, when all through the house, not a creature was stirring, not even a mouse.
‘Twas is a contraction of it was, and the rule with contractions is that we use the apostrophe to stand in for missing letters.
For more holiday grammar fun, see my 10-question quiz: ‘Tis (or t’is?) the season. I bet you’ll get the title question correct, at the very least.
How Stuff Works includes A Visit From Saint Nicholas in its list of the top ten myths of Christmas. Apparently, although Clement Moore is usually credited as the author, he probably was not.