The creators of That 70's Show had decided on setting the show in 1976 so that it would feel more rooted in the ideals and trends of the 70's. What they didn't count on, however, was the show being so successful that it would last for eight full years! Evidently, as the series went on, the actors began to get much older and the writers needed to slow down the show's time line.
This created problems because most seasons had a Thanksgiving and Christmas episode, which lead to more holidays than the four year timeline should have had. Because of the compressed timeline, many events that should have been months apart were actually weeks or days apart. What's neat is that if each season had been one year, the show would have ended in 1984, which was the year that "That 80's Show" took place.