Brian Wilson's father, Murry Wilson, sold the rights to the Beach Boys catalog in 1969 for a tiny fraction of their ultimate worth. Though Brian went ballistic when he found out, claiming that his signature was forged on the documents, Murry was sure he made the right decision. The Beach Boys were old news in 1969, and he felt a bunch of vintage surfing songs were almost worthless. Four years later George Lucas released American Graffiti, an uber-nostalgic movie about a bunch of teenagers in 1962. The soundtrack, full of music from the era, in many ways kick-started the endless wave of nostalgia for the time. The movie ends with "All Summer Long" by the Beach Boys. The song is from 1964, and it's quite possible that Lucas simply made a mistake, but the film ends with the characters looking forward, and it made sense to play a song from a slightly later period.
The supremely joyful song is the opposite of "Summertime Blues." It's about the endless joy and wonder young people can find in summer. It's far from the Beach Boys' biggest hit from their early days, but it's certainly one of the best. Anyone that lived through the time will be brought right back to 1964 after listening to it. Like many Beach Boys songs, it's earned the rights holders a large fortune over the past 50 years. Murry Wilson was many things, but he wasn't much of a forward thinker.