The Great Migration moves through the Serengeti and Masai Mara year round, and the best time to see it depends on what you want to see.
From December to March, the herds gather on the southern plains for calving season. By June, they are moving north through the western corridor, crossing the Grumeti River. By July through October, the drama shifts to the Mara River crossings, widely considered the most spectacular part of the migration.
There is no single wrong time to visit. There is only a different chapter of the same story.