Cannabis is the most commonly used federally illicit drug, yet little research has identified risk factors for high-risk use. Although April 20 (“4/20”) has been identified as a day on which cannabis users use more cannabis than they typically do, it remains unknown whether 4/20 is a high-risk event for greater use than other heavy use days (e.g., other holidays) and whether use on 4/20 is related to greater use-related problems. Such information has important implications for event-specific prevention efforts. Thus, the present study tested whether 4/20 is associated with greater cannabis use than other high-risk cannabis use events (e.g., Spring Break), whether 4/20 cannabis use is associated with event-specific use-related problems and subjective intoxication, and whether event-specific psychosocial factors (i.e., motives, normative beliefs) influence 4/20 use among 68 past-year cannabis using undergraduates who reported using cannabis on 4/20 (in a state where cannabis is legal for medical but not recreational purposes). A majority reported experiencing problems related to their 4/20 use and use was greater on 4/20 than on all other high-risk days. Quantity of 4/20 use was significantly, positively correlated with 4/20 cannabis use-related problems and subjective intoxication. 4/20-specific descriptive norms were a stronger predictor of 4/20 cannabis use and use-related problems than 4/20-specific cannabis use motives whereas 4/20-specific enjoyment motives were stronger predictors of subjective intoxication on 4/20. The current study identifies 4/20 as an especially high-risk cannabis use event and is the first to test psychosocial factors related to 4/20 use and use-related problems. Event-specific descriptive norms and enjoyment motives may be important targets for campus-wide efforts to prevent risky use on this high-use day.