The great migration through the Serengeti-Mara is one of the world’s most iconic wildlife spectacles. The only surviving mammalian migration of comparable scale, it comprises some two million grazers – mostly wildebeest but also zebra and gazelle – that follow a 600-mile annual clockwise loop through an unfenced grassland ecosystem shared between Tanzania and Kenya. The migration is truly sensational to witness, whether you catch the wildebeest marching in formation like a horizonless column of giant army ants, or undertaking a thrilling river crossing, or calving en mass while a coterie of carnivores waits to pounce on the defenseless newborns.

It’s fair to say that catching the migration is a high priority on the wish list of most visitors to East Africa. However, it isn’t as straightforward as just pitching up and hoping for the best. Although the migration follows a fairly predictable annual pattern, nothing is set in stone, and there might be considerable variation from one year to the next depending on rainfall and other climatic vagaries. Furthermore, the drama unfolds on a stage of vast proportions. Extending eastward from Lake Victoria (Africa’s largest lake), the Serengeti-Mara comprises 11,500 square miles of prime safari territory, including Tanzania’s Serengeti National Park and Ngorongoro Conservation Area, Kenya’s Masai Mara National Reserve, and several smaller government, private and community game reserves.

To stand a decent chance of catching the migration, you need to be in the right place at the right time, or you’ll likely find out the hard way just how elusive a herd of two million animals can be. Fortunately, even if you do get it wrong, Serengeti, Ngorongoro and Masai Mara all rank among Africa’s most consistently rewarding safari destinations, offering excellent game-viewing throughout the year, with a good chance of spotting all the Big Five (lion, leopard, elephant, buffalo and rhino) together with other safari favorites from giraffe and hippo to cheetah and spotted hyena. Nevertheless, reading the crib notes below, before you start planning an itinerary, will give you the best chance of coming across the wildebeest in action.

Although the migration is a cyclic process, the tail end of the rainy season is its logical start point. Late May or early June is when the wildebeest start to amass in the southwest Serengeti, moving northward in burgeoning herds that occasionally turn back after a major storm, eventually arriving in the acacia-studded savanna of the Western Corridor towards the end of June. The defining action of this northward surge is the adrenalin-charged crossing of the Grumeti River. This is a one-off annual event most normally preceded by several nervy days of mass indecisiveness before one or two bold pioneers hurtle across the river, and the rest follow, providing a gory feast to the massive crocodiles that lurk in the muddy waters.

Arguably the most reliably exciting time to be in Serengeti-Mara, August and September is when the wildebeest disperse across the undulating northwestern plains following a dramatic crossing of the Mara River. Unlike the migration across the Grumeti, however, the Mara crossing is not so much a one-off event as a succession of backward-and-forward movements dictated by fluctuations in rainfall distribution. Each of these local crossings – which might comprise anything from a few hundred individuals to tens of thousands – is an adrenaline-charged spectacle in its own right, and potentially perilous for the participants, hundreds of which die annually in misjudged high-water crossings or are taken by the crocodiles that live in the river or lions that wait on the opposite shore.

In late September or October, the wildebeest will cross the Mara River one last time before setting off on a month-long southward trek. This more-or-less follows the eastern border of Serengeti National Park, arrive on the short grass plains of the Serengeti-Ngorongoro border area in time for the November start to the rains.

December and January generally form the most uneventful and sedentary phase of the migration as the wildebeest disperse into the plains that stretch southeast from Seronera Park Headquarters into Ngorongoro Conservation Area. Static as the wildebeest might be during this phase, they are still often encountered in impressive herds of up to 10,000 individuals.

Vying with August/September as the most thrilling time to be in the Serengeti, February is peak calving season for the wildebeest, which remain dispersed across the southeastern plains of the Ngorongoro border area. Hundreds, even thousands, of wildebeest calves are born daily, replenishing kin that perished during the Mara and Grumeti crossings. Sensitive safarigoers be warned that the wobbly-legged wildebeest newborns provide rich pickings to the many lions, cheetahs and other carnivores that concentrate in the area.

Although calving activity dies down by early March, the wildebeest usually remain in the southeast Serengeti for another three months, raising their young in preparation for the northwestern surge. You won’t see the migration in full swing between March and May, but you might well catch herds of several thousand wildebeest moving locally to greener pastures. Despite being the rainiest time of year, April and May are also when tourist volumes are at their lowest, rates are most competitive, and the scenery is at its most photogenically verdant.