But it has a 20th day, which is a better joke. You can call it 20/4 if you want, but we like to call it 4/20.
It is certainly important to know who is writing dates. If a gringo writes 01-04-05, it means the fourth of January 2004. If a German writes 01-04-05, it means the first of April 2005. If a Chinese person writes 01-04-05, it means the fifth of April 2001.
It's all good. Pi Day is still Pi day, no matter how your write it, and it happens on the 14th of March. Mole Day, by the way, happens on the 23rd of October. Of course if you write those dates the gringo way it makes more sense.