To determine in which month the populations of Species A and Species B were most similar, we would need population data for each species across the months mentioned. Since I don't have that data, I can't definitively tell you which month had the most similar populations.
If you have access to the population figures for Species A and B over the months, you can compare the numbers for each month and see where they are closest together. Please provide the data if you would like assistance analyzing it!