If you've recently planted grass seed, cutting a newly sown lawn too early can halt its development resulting in a sparse and patchy lawn. Make sure you wait until the grass blades have grown to at least 2 inches before mowing for the first time. Set mower blades high to avoid stressing the plants and allow a rest period of 5-7 days before repeating.
Cutting the grass too early or too late in the day can also cause problems. Mowing too early when dew is still on the ground can cause the mower blades to clog up and grass to be pulled rather than cut. A cut in late afternoon or evening can lead to fungal spores taking hold and developing overnight.