Once the seedlings have grown to height of 3 to 4 inches, they can be mowed for the first time.  Your mower should be in good condition, and the mower blade, newly sharpened .  Remember that we are dealing with a very young and tender plant with a new, root system.  A mower in poor condition can uproot the seedling, or damage the tender grass blade.  DO NOT mow when the soil is moist.  Your footprints and mower wheels will sink into the soil and you could damage or uproot the seedlings.  Once conditions are right for mowing, cut the grass at a height which you plan to keep it at.  Recommended mowing height is 2 1/2 to 3 inches.
Mowing frequency should be governed by the lawn’s growth.  Mow frequently enough as not to remove more than 1/3 of the grass blade at a time.  Cutting too short or cutting off too much of the grass plant at one time may shock the grass plant and retard its thickening for 6 to 8 weeks, and cause your lawn to look yellow or even brown.  Do not bag clippings, unless they will result in clumps and wind rows on your lawn.