The Great Migration isn't a single event, it's a year-round circuit of roughly two million wildebeest and zebra moving through the Serengeti-Mara ecosystem in search of rain-fed grass. Understanding roughly where that circuit places the herds in a given month is the single biggest factor in planning a migration-focused trip well.
December through March generally finds the herds calving on the short-grass plains of the southern Serengeti, which means enormous concentrations of animals and predators working the newborns. By June and July the herds are pushing north through the western corridor, and the famous, chaotic river crossings typically play out from July through October, mostly in Kenya's Mara but sometimes still on the Tanzanian side depending on the year.
No operator can promise a crossing on a specific date, because the animals genuinely don't read the calendar. What we can do is put you in the right region during the right window and keep the itinerary flexible enough to reposition if the herds move faster or slower than usual that season.