Trick question: there are none, you slacker!

Actually, there are a couple of excuses that may be valid on a case-by-case basis:

Hospital emergency
Probably any other emergency
Dentist appointment
Dog ate your homework
You’re performing at the Melbourne Comedy Festival Gala
You would have loved to come but just during work today you started to get a sore throat and probably it’s best you rest tonight as you think you might be sick, but you’re keen to catch up sometime soon and so sorry for the last minute bail, but let’s grab lunch next week at work if you’re around and so sorry again because it would’ve been great to catch-up tonight
Cyclone Alfred
You’re going to Heathrow airport to run through security and say goodbye to your one true love because she is going to America
Covid-19

  If none of the above applies to you, get a ticket mate!