I pick 13, because so many people hate the #13 for insane reasons. They even created a false medical diagnoses for it that is supposedly a treatment for the badluck and fear of the #13. paraskevidekatriaphobia If you can pronounce it correctly supposedly your fear & bad luck mysteriously goes away... Rofl.
But I pick friday the 13th as my luckiest day because my two largest Casino hits at the slot machines were on Friday the 13th. One for almost ~1k USD.
Do I believe in superstition? No. I believe the RNG of the casino coughed up a hit on my machine which in turn paid off big for ME and allowed me to buy some expensive toys later