The reason is that the number 420 (also known as April 20, which is 4/20 in American spelling) has a “long history” related to marijuana as well as many other troubles.

Since 1971, April 20 has become international marijuana day and has been recorded in many songs, movies and other forms of culture. The reason is that, at 4:20 pm on April 20, 1971, a large group of young people from San Rafael High School, California gathered in an empty lot to smoke marijuana together.

Since then, 420 has become a favorite number and even a code for marijuana smokers. To celebrate this holiday, every April 20, marijuana lovers have a smoky celebration. The most common is that they will go to hotels to steal the room number 420 sign as a souvenir. Because they keep hanging it up and losing it, many hotels have finally decided to remove the room number 420 altogether.