If you have cool-season grasses, a minimum of two fertilizer applications is recommended. For spring and fall, one is enough. It can be more though, depending on the condition of the lawn. If you choose to fertilize one time a year, the fall is the ideal period for cool-season grasses. If you have warm-season grass it requires regular fertilization at the time of heaviest growths which is usually in the summer. The application of fertilizer for warm-season grasses depends on the type of soil like clay versus sandy soils, etc. It also depends on how well the nutrients are retained in the soil.