What holiday is 4/20

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

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.