Fertilize perennials now -- it's not too late, but next year, do it earlier.

Fertilize your grass in the fall.

Fairfax County Extension recs fall fertilization. If you must in the spring, wait until the grass is growing again (early May) and apply Nitrogen at half the rate.

http://www.ext.vt.edu/departments/envirohort/factsheets3/lawn/MAR88PR5.HTML

You're not supposed to fertilize lawns until fall--you can do it at half the rate as biddy67 mentioned around May 15 or so. I just signed up for the Great Scapes program in Prince William county. They send someone out who takes a soil sample and measures your lawn and then gets back to you with an analysis of what you should do when. Fairfax may have a program similar.

I do my fertiliing any time I want to but then again I cheat and use worm castings and they can be put out any time because it dont burn. I am the kind of person that can burn up a lawn by thinking of fertilizing so now I only use worm castings because even I cant screw it up. Just remember that too much and you will be cutting the most lush greenest grass about every 4 days.