When does the annual expat exchange tend to happen, does it happen June/July or September for the schools? Guess this a great time to find properties and cars?

The majority of expats that are leaving with kids depart Dubai at the end of June/beginning of July as that is when the summer term finishes so yes this is a good time to find properties and buy bargain cars.  By September it is getting busy as new expats are arriving and looking for homes and cars.