The Wildebeest Migration in June happens to be one of the fastest, as the herds migrate fast to northern Kenya due to the rains experienced in the north. The Serengeti and Masai Mara all experience a good amount of rainfall, making it attract the gnus to these regions.
They won’t stop for long at any given point, unless, a good amount of rainfall falls in Central Serengeti or Grumeti regions.