From where did the band Green Day get their name?

In 1987, friends Billie Joe Armstrong and Mike Dirnt, 15 years old at the time, formed a band called Sweet Children. Its first live performance took place on October 17, 1987 at Rod's Hickory Pit in Vallejo, California where Armstrong's mother was working. In 1988, Armstrong and Dirnt began working with Sean Hughes and the former Isocracy drummer John Kiffmeyer, also known as Al Sobrante. Kiffmeyer served as both the band's drummer and business manager, handling the booking of performances and helping the band establish a fan base, and Sean Hughes served as the band's bassist. As said in the film Punk's Not Dead, Armstrong cites the band Operation Ivy (which featured Tim Armstrong and Matt Freeman of Rancid) as a major influence, and a group that inspired him to form a band.

After Hughes left Green Day in 1988, Larry Livermore, owner of Lookout! Records, saw the band play an early show and signed the group to his label. In 1989, the band recorded its debut extended play, 1,000 Hours. Before 1,000 Hours was released, the group dropped the name Sweet Children; according to Livermore, this was done to avoid confusion with another local band Sweet Baby. The band adopted the name Green Day, allegedly due to their fondness for marijuana.

https://en.wikipedia.org/wiki/Green_Day

Green Day -- "green day" is a slang term for spending a day smoking marijuana. Billie Joe Armstrong wrote a song called "Green Day" about his first experience with the drug, and it soon replaced "Sweet Children" as the band's name.

https://en.wikipedia.org/wiki/List_of_band_name_etymologies#G

At one time, all members of Green Day used marijuana regularly. As a means to celebrate their favorite drug, they named their band after a day smoking weed. It is also a common misinterpretation that they came up with the name for the band when Mike Dirnt and Billie Joe Armstrong dropped out of school to pursue their music career, their school principle told them "It will be a green day in hell." However, this myth has been proved untrue. When Green Day first formed they had the name "Sweet Children", but changed to Green Day as a way of showing that they were not kids anymore.

It's an east bay expression about smoking pot all day- well something like that. "Green day" is a day spent doing nothing but smoking weed. The guys were watching Sesame Street (don't ask me why) and they heard Ernie say "Today is a green day!" This was funny to them, so one of their songs was soon titled Green Day. They later decided to switch names from Sweet Children to Green Day.

http://www.amiright.com/names/origins/g_1.shtml