The most important thing you can do for a lawn is provide it with proper
nutrition. Fertilizing is probably the cheapest and easiest way to make
a big impact on your lawn. 
There are various ways of feeding a lawn and depending on which option you select depends on how often it should be fertilized.
How you handle your grass clippings also makes a difference.
If you bag your grass clippings, you're depriving your lawn of an abundance of free nutrients that must be replaced by the grass plant extracting more elements from the soil and depleting those resources faster.
If you opt for an organic approach, then you won't have to fertilize as often as you might using granular or liquid fertilizer mixes, but the application will cost more and take longer for effects to become visible.
If the grasses appetite isn't fed, the the lawn doesn't have the resources to stand up to normal turf stresses that popup throughout the growing season. If you opt to only feed your lawn 1 — 2 times a year, you're starving the lawn. A steady regular diet (not over feeding) is required.
Fertilizing means you have to mow your lawn more often
Fact: Don't use quick release nitrogen fertilizers. Use time release or organic fertilizers. Look at the label for descriptions such as: extended feeding or slow-release nitrogen. Slow release fertilizers will spread out the lawn feeding over a longer period of time and will be less likely to cause a burst in grass growth.
Early in the morning when the dew is heavy. The dew means you'll leave tracks walking across the lawn and it'll be easier to see where you've been. Also, there is less wind and the fertilizer is less likely to blow around.
Early fall is the most important time of year to fertilize your lawn. Think of the early fall lawn feeding as the big dinner. Spring is breakfast for the lawn and late spring is a light mid-morning snack. Lunch is usually eaten in late summer.
Nitrogen: (N) Boosts growth and green color of turf
Phosphorous: (P) Feeds grass seedlings and turf root development
Potassium: (K) Strengthens a lawn's resistance to drought and disease
