Jesse McKinley, Marijuana Advocates Point to Signs of Change, in The New York Times, 19 aprile 2009. URL consultato il 23 gennaio 2011 (archiviato il 20 gennaio 2011).
«Mr. Hager said the significance of April 20 dates to a ritual begun in the early 1970s in which a group of Northern California teenagers smoked cannabis every day at 4:20 a.m. Word of the ritual spread and expanded to a yearly event in various places. Soon, cannabis aficionados were using "420" as a code for smoking and using it as a sign-off on flyers for concerts where the drug would be plentiful. In recent years, the April 20 events have become so widespread that several colleges have discouraged students from participating.»
