What holiday is 4/20

April 20 has become an international counterculture holiday, where people gather to celebrate and consume cannabis.

Explanation:

... On that day many marijuana users protest in civil disobedience by gathering in public to light up at 4:20 pm.

Answer: It's not an official holiday but it is a day where across the globe people participate in the use of marijuana products either medical recreational or black market marijuana black market meaning off the street