You'll have to imagine the cut out letters from magazines.

Dear Advent Calendar Day 11

We have Advent Calendar Day 10.

If you ever want to see Advent Calendar Day 10 again,  well... you're out of luck since time moves in a linear progression.

Unless you're a Time Lord of course, in which case attempting to blackmail you is probably a poorly thought out course of action.

Please disregard

Love,

No one in particular.