We cannot say for sure what the problem is. We do not have enough information how your turf has been managed. Some reasons for the brown spots may be planting the wrong turfgrass species, poor soils, fertilizer injury, close mowing, grubs, brown patch, etc. See our website for more information http://extension.umd.edu/hgic/cultural-environmental-problems-lawns
Turf type tall fescue is the recommended grass species and grows best in full sun to part shade. If you planted a contractors mix of different species at some point, some of the grasses may be dying out. Also, Some grass species can go dormant at different rates during hot, dry weather. Poor soils - not enough organic matter in the soils to promote root growth, or close mowing.
Lastly, Brown patch is a common summer disease of cool season turfgrasses, June - September. Temperature and humidity favor this disease. Avoid spring applications of fertilizer. Apply fertilizer in the fall. Most turf recovers in the fall with a return to rainfall and the proper fall fertilization.
See more on brown patch http://extension.umd.edu/hgic/brown-patch-disease-maryland-home-lawns
Sounds like you may have to do some spot renovation or overseeding in the spring.
Test your soil if not done in the last several years. Results give pH, liming, and fertilization recommendations. https://extension.umd.edu/hgic/soils/soil-testing
See our publications on lawn renovation and overseeding and our turfgrass maintenance calendar for more information on maintenance and care throughout the year.
You can send us photos in the spring if need be.