Riddle: 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 this teaser referring to?
A:N/ Really , REALLY HARD! Took me FOREVER!! First to solve gets a dedication and anything else you want. Luck! :D :D