Chronologically, I assume the Storm King's invasion of Equestria and the pony nation's libetation by the Mane 6 and their allies take place between the end of the television series' 7th season and the beginning of the 8th season.

Bonus question: does the Storm King return after the end of the MLP movie? Guess: he teams up with Chrysalis, a group of unreformed changelings, Lord Tirek, and several ancient monsters fron Tartarus to threaten Equestria.