We have collected over 5 years of Masai Mara River crossing data and for a given set of dates, we calculate something called Migration Index (MI), which says what was the % days that migration happened. 100% means all days the migration/crossings happened and say of the 10 days you have given, only one day it happens, them the migration index is 1/10 = 10%.
Put your proposed tour dates for the Mara crossing and we will put the Migration index for those days but for the last 5 years and aggregated Migration Index for all 5 years combined and you will get a feeling of what might be your chances of seeing the migration this year from Historical data.
PS: This is just a statistical & historical figure and might/might not give whether a crossing might happen but helps to get what happened during those days in the past five years. Please use the data with caution as we @ WildTrails gives no guarantee on the actual happenings of crossings as it depends on various factors (and we in our labs are continuously adding more variables to get a very predictive metric - stay tuned)