First happened on January 1, 1976.

http://i.imgur.com/c2z9lGb.jpg

On Jan. 1, 1976, the same day California’s relaxed marijuana law took effect. Cal State Northridge student Danny Finegood altered the Hollywood sign to read Hollyweed by using $50 of curtains and a couple friends. Later in 1976 they had it read “Holywood” on Easter. In 1987, they protested Marine Lt. Col. Oliver North amid the Iran-Contra hearings with “Ollywood.” In 1990, it read “Oil War” in a protest of the Persian Gulf War.

Tired of what they saw as vandalism, city officials eventually beefed up security with a fence, alarms and eventually installed a closed-circuit surveillance system. With the increased security, Finegood said “it was clear they wanted to keep people away from the sign,” and decided to respect that by ceasing his alterations.

Here is another view.

Several other versions of alterations may be seen here. These include Raffeysod (which no one seems to understand), Caltech, Perotwood, JollyGood and SaveThePeak.

Source.