We experienced them in July.........stay away if at all possible.
They were everywhere, could not get away from them at the park, going to a different park or water parks.
We stayed at the Poly so that was a relief from the Big Tour Groups.
We went during the second-third week of August and there were no BTGs. It was so much nicer for crowds.
we were just there in mid May - I don't remember seeing any of the 'foreign' tour groups but there were tons of school kids there (in organized school groups) for what I'm assuming was some kind of end of year trip.
This is easy to remember... they start on the 1st of January and go until the 31st of December. Some months are heavier then others, but, they are pretty much year round.