Whats a good excuse for me to take the day off work tomorrow?

You have mondayitis. It's a beautiful day and why not celebrate you? Heck, you woke up and felt like going to a coffeeshop to read the paper and plan out your day. Do it. At my last position, a manager told me that she takes 'mental health days'. Ask yourself-will your company shut down if you aren't there one day?

Sounds like somebody has a case of the Mondays!

Migraine

you ate the wrong thing at a restaurant last night and your stomach is upset (a nice way of saying you have diarrhea). no boss could argue with that.

Tell your boss you are feeling a bit Homicidal or postal! It his duty to protect the other employees.

Use the phrase, “I was doing the reverse pivot all night” and nobody will ask for any more details.