Crossings cluster late July through September, with the single highest-probability window running mid-August to mid-September — but 'probability,' not certainty, is the honest word, since a crossing depends on herds actually reaching the river's edge in numbers and building the nervous momentum that tips them into the water, and that can happen on any given day or not at all for a week. The northern Serengeti's Mara River crossing points (around Kogatende) tend to be calmer and less crowded than the Kenyan side's famous crossings near the Mara Triangle; the Kenyan crossings draw more vehicles but are marginally more reliable in peak weeks because the herd concentration is denser there by late August. Multi-night stays beat single-night stops badly here — three or four nights at a camp positioned near an active crossing point gives your guide the flexibility to reposition daily based on herd movement reports shared across camps each morning, whereas a one-night stop is a coin flip. If witnessing a crossing is your primary goal rather than a nice-to-have, build your entire itinerary's timing around it and treat everything else as secondary.