The April 20 dates back to the 1970's when a group of Northern California teenagers smoked marijuana daily at 4:20 p.m., The New York Times reported Monday.

As the ritual spread in this subculture, marijuana fans began using 420 as the code for smoking pot and designated April 20 as a day of celebration.

Police said they would be on heightened alert Monday for people reveling in 420 celebrations. In Kansas City, for example, police planned to saturate the streets looking for drivers under the influence of marijuana.

“People need to recognize that a DUI is not just from alcohol,” Sgt. Daniel Graves with Kansas City Police Department’s Traffic Investigation Unit said in a statement.

Last year, Kansas City police arrested 70 people for driving under the influence in cases that involved marijuana or other drug use.

The New York Times