For the growing season. Keeping your lawn too short will stress it out, especially in the hot dry mid-summer period. Too short also promotes invasive weeds to take hold. Keep the grass height somewhere between 2-2 1/2 inches or (5-6 1/4 cm). Mow and mow often with a sharp blade. Decomposition of the shorter cutting will not build heavy thatch. Cutting long grass will leave excess material which should be bagged instead of letting it build a heavy thatch layer. A heavy thatch layer settles at ground level and chokes out air ,sun, and nutrients from penetrating the soil to the roots system and will cause problems.
For winter dormancy. Your mowing program should start in the fall. In fact this is the key to successful turf management and in turn promises a potentially trouble-free spring startup. Fall mowing heights are very important for the turf to survive the winter.
If you leave the grass too long and it lays over under the heavy snowfall it is prone to getting snow mould. As the snow melts away the flattened grass will start to mould, killing/smothering the area. A healthy mow height of 2-2 1/2 inches or (5-6 1/4 cm) is best. Progressively lowering the final height with 2-3 cuts is essential, because cutting off too much at once stresses the plant out.