As a general rule we fertilize 3 to 4 times a year. (Easter, Memorial Day, Labor Day, and Halloween). If you choose to fertilize once, do it in the fall. If you choose to fertilize twice, do it in the fall and spring.
Bluegrass is a wonderful plant! Humans, through management, have changed the way the grass first grew. Originally, the grass was a "clump" grass growing in pastures. As time passed, with additional water and nutrients, the plant grew more closely together making a better lawn.
Its natural growth cycle was to be green in the spring with natural moisture, go dormant during the dry summer, and green up again with fall rains. However, people aren't satisfied with this cycle. We want it to green early in the spring, and stay green until fall. A dormant lawn in the summer isn't acceptable!