September is the time for the most important fertilizer application of the year. Use a slow-release high-nitrogen fertilizer sometime in the first half of the month. This helps the lawn recover from the stress of the summer heat.
September is also the time to begin cutting back your watering frequency as temperatures cool down. Keep an eye on daily temperatures to determine when it makes sense to add another day between waterings.
And, September is also a great time to core aerate the lawn. This may or may not need to be done each year, depending on the amount of use your lawn gets. Be sure to leave the cores on the lawn after aerating.