4:20 is code for smoking weed. It comes from the time a bunch of 1970s high school students got together to blaze, and sort of grew into a general reference to a time to get high.
You can still see people on social media posting "420" or "4:20", as a sign that it's time for everyone to light up.
From the time 4:20 came the date 4/20, which is now informally celebrated as National Marijuana Day. There are some celebrations around the US, in places where weed is legal. As you might suspect, people celebrate by getting together and getting high.