Hi from a fellow Brit (who doesn't live there anymore)
It's hard to say why most of the world says day then month (20/4) while those across the pond say month then day (4/20) Wikipedia has info on why 4/20 is significant for puffers. We could also ask why they are the only country to still use Fahrenheit ?
But who needs an excuse or a time to smoke the weed anyway?
Take Care