Brennan Linsley / AP

Downtown Denver's 4/20 fun included marijuana-related merchandise and music, with rapper Rick Ross performing at 4:20 p.m. MT on Sunday.

No official event was planned for Monday — 4/20 itself — at the Civic Center. The University of Colorado Boulder also announced it would operate as normal on Monday. For the last three years, the school has shut down on 4/20 to avoid "disruptive gatherings," chancellor Philip P. DiStefano said in a letter to students. The lawn area of the campus' Norlin Quad would still be closed, he added.

"We have made great strides over the past three years, and I thank all of you for your patience and cooperation in helping us toward that goal," he said.