It's important to adjust watering habits in the fall. Come late September, the nights are much cooler, the heatwaves are over, and the soil has the ability to stay moist longer. As a result, the water requirements of the summer are no longer present. Continuing to water the grass as you would in the summer would result in weakened, waterlogged turf.
Rainfall is more effective in the fall than in the summer. Depending on the weather, an inch of rain could be enough water for two or three weeks, by which time it may have rained again. Supplemental water with irrigation or sprinklers is hardly necessary unless there's an all out drought.
Monitor soil moisture and only water when necessary. Over-watering at this time of year will only set the lawn back rather than keep it healthy. Don't become your own worst enemy by over-watering.
