Well, it is a sensible time to smoke pot. I've probably smoked my fair share in my time. You can't smoke in the morning because you have too much to get done... although I know a remarkable few individuals who lead surprisingly productive lives in a perpetual haze of pot smoke. A joint at 4/20, a quick (and large dinner), and you'll be fine to deal with whatever you have to do at night.
In Manila where I spent most of my youth, we do the Spanish siesta thing around 4/20, nap or joint... really up to you. But this doesn't really have anything to do with music, does it?