So yes, the planes are empty, but September is the timeframe to watch.
Maintenance costs are based on different schedules, there is usually a time limit (as in calendar time), a flight time limit and a cycles limit (a cycle is one take-off + landing). Which one is more limiting depends on the type of operation the plane was designed for, A long-haul plane is designed for more flight-time vs cycles, a regional jet is designed for a high cycle count. So maintenance costs will decrease if you keep them on the ground, but some components may then "time-out" which will still incur maintenance costs to bring them back to airworthy condition.