Fertilizing in mid-May is an ideal time because root growth is active and eager to pick up nutrients. There are several types of slow release nitrogen and the best ones would be IBDU, Isobutylidenediurea , or polymer coated nitrogen. These products will spread the release of nitrogen over three months so you don't need to fertilize again until late summer. Mid to late August would be the next application, followed by the late fall application in mid-October. If weeds continue to be a problem or become a new problem due to difficult summer conditions, a weed control application in September will effectively remove the weeds and allow for the grass to thicken up before winter.
Weeds are opportunistic plants which follow human disturbance and invade whenever they get a chance.
By preventing their invasion in the first place, your image of a beautiful garden will be much more easily realized. Daisy Moore, 1999.