Colorado has heavy clay soils which can suffocate grass roots. We advise semi-annual aeration in the Spring and Fall. However, if you choose to only aerate once a year, do it in the Fall. After a summer of trampling, the grass it could probably use help decompacting. Also, root growth for turf grass is very active in the Fall as the grass prepares itself to go dormant. By aerating during this time the holes left by aeration can be filled with new roots.