"When is the migration in the Mara" is the single most common question Kenya-bound travellers search, and the honest answer is: it depends which month you're asking about, because the herds are moving constantly through a year-long circuit.
Rough month-by-month
- July–August: herds arrive in the Mara from the Serengeti, river crossings begin
- September: peak river-crossing season at the Mara and Talek rivers
- October–November: herds start drifting back south toward Tanzania
- December–June: herds are largely in the Serengeti, calving in Feb–March
If a dramatic river crossing is the goal, July through September is the safest bet, though nothing in nature is guaranteed on an exact date — the herds move on rainfall, not a calendar.