Number one reason is because you don't think this day will come. "Oh, it will never be April 6th. The world will end on April 5th."
Reason number two is because you hope it will go better than last time. "Last time I made one mistake per page. This time maybe I'll do one in two pages."
And the last reason is because you hope nobody will really notice your mistakes. "Let's hope my colleagues won't show up. Member's of the congregation can't tell a difference between Bach and Offenbach anyway."