The best time to fertilize is when you aerate.  This will allow the nutrients from the fertilizer to go deeper into the roots of your grass as you water it in.  Less fertilizer will run off and be caught in all of the holes that the core aeration leaves behind.  This will give you optimum results if you water properly.  It is also important to fertilize in the summer, that's why Happy Roots offers multiple fertilizer programs.  Feeding your lawn in the summer keeps the canopy of your turf grass thick to keep weeds out and retain more of the water you are putting down.  Our 4 and 5 application fertilizer programs are designed to give your lawn optimal feeding intervals to achieve the results you want.