In moderation I have no real problem with weed. It's no more/less detrimental to your health than alcohol/tobacco/fast food/etc. is. Matt Stone and Trey Parker pretty much hit the nail on the head when they said that drugs are not a good idea because they make feeling bored ok, when what you should be doing when you're bored is pursuing interests, learning new things, developing skills, etc. Weed is also incredibly addictive, not in the chemical sense like nicotine, but just because it is effortlessly "fun."
Your best bet is to stay away from them entirely so you don't know what you're "missing out" on, because in the long run you'll be thankful you did. If you've never done it though, you should show some sympathy to those that are hooked. Their addiction to weed is no different than our addictions to videogames. If someone told you to quit playing videogames for a year you probably couldn't do it. The same goes for someone who gets high.
4/20 day is retarded though. Celebrating drug dependencies is kind of ridiculous. I have a lot of friends who look forward to this day for a long time, but I honestly find it rather sad.