My First is in Yesterday, But not in Today

My first is in yesterday, but not in today;
My second is in after, but not in before;
My third is in start, and also in stop;
My fourth is the 20th letter of the English alphabet;
My fifth is in correct but not in wrong;
and my sixth is in pretty, but not in cute.
My whole is something celebrated by Christians.
What is the teaser referring to?

Click here for Answer