It may seem sometimes that spring rains just won't let up in time for a spring application of fertilizer. It's far better, however, to skip a round of fertilizer application altogether than to risk burning your lawn's tender new growth while it's recovering from winter. Applying some kinds of fertilizer onto wet grass does more harm than good to your lawn.
Chemical lawn fertilizer comes in granular and liquid form. Most have high nitrogen content to give the lawn a thick, intense green appearance. Some include iron to help strengthen and further green up grass. The nitrogen and the iron in these formulas are the things that can do damage to the lawn when applied incorrectly. If applied to grass while the blades are wet, granular formulas will stick to the blades.
Once stuck, even thorough watering may not be enough to shake the granules loose. Likewise, if water is briefly applied to the grass after fertilizing, any granules that haven't managed to sift down off leaf blades may get stuck. The blades of the leaf are exposed to high concentrations of nitrogen and/or iron, and start to "burn" from chemical exposure. The damage may show up as mottled blades, or even as entire patches of yellowing or brown burnt lawn.
Once the spring rains let up, wait until all moisture (including morning dew) is gone from the leaf blades. On a series of cool days with heavy cloud cover, this could be a day or so. If the soil is lightly moist, this is fine. But if it's soggy, the fertilizer will just run off. After applying granular fertilizer, water slowly over the entire lawn to make sure the granules wash off the blades, but don't over-soak the grass. This will carry the fertilizer off the lawn and into the storm water system, where it becomes pollution.
Manure, alfalfa meal, humus and compost can also be used to fertilize the lawn. While it's still better to apply these to a dry lawn and then water them in, the chances of burning and the severity of the burn are much less, especially if you try to rinse it off of the wet blades afterward. And while your grass may not always need the high to excessively high amounts of nitrogen in most chemical lawn fertilizers, the trace elements and microbe-richness of organic fertilizers is a boon.
Liquid fertilizers won't burn the grass if they're applied to wet blades. In fact, they do well when applied to damp soil, as the moisture in the soil helps move the nutrients down to the root system. Again, excessively soggy soil will create runoff problems, so you should wait unit it's had a chance to dry out a bit. Granular fertilizer should always be watered in after being applied to grass, because prolonged exposure to the dry grass blades may still cause burns.