While the exact time line of "That '70s Show" is rarely taken literally, theoretically-speaking, the Led Zeppelin concert that Fez and Caroline likely attended would have been the Chicago series of concerts, which took place in early April, 1977.
Prior to the opening scene on the DVD version, Wilmer Valderrama gives an intro and he states that this was among his favorites to film.
In Red's future dream sequence, he is employee of the month. The sign says, "33 years of dedicated service". He tells Eric, "You're 57 years old". Red & Eric started at Price Mart within days of each other. Eric was 17 at the time he started, which would make him 50 during the dream.