The movie's phrase is "Season of the Witch" and it was an attempt (and I am guessing) at making the franchise be about more than the killer Michael Myers.
This was a bold step because the fans of the franchise in 1982 quite liked the previous 2 films which of course, centered around Micahel Myers.
The movie isn't horrible, it just wasn't what the fans were looking for. It would be like going to see Nightmare on Elm Street only to discover that Freddy isn't even in the film or Friday the 13th with no Jason Vorhees. It doesn't make sense.
John Carpenter also stepped away from directing although he did serve as a producer. I can find no record of what he thought of not including Mike Myers in the script, but i would imagine that he would now say he was not pleased with it.
The story focuses around a costume manufacturer who has a series of popular masks that contain some sort of magical powers that will be activated when the wearers watch some sort of magical television activation. It is still creepy, especially by 1982 standards, but I rewatch this movie only for the "what the hell were they thinking" moments. The only reference to MIchael Myers is presented in a reference to fictional stories inside the movie and only briefly at that.
overall this movie is a pretty important part of history only because it didn't make much sense to change an already successful system into something that was unproven. The movie still managed to make a decent profit, but was met with mostly negative critical review and fan-fare. It has its moments, but they are few and far between.