For cool season grasses (fescues, Kentucky bluegrass, bentgrass) you can fertilize once in the late spring and twice in the fall. Use the fertilizers advertised for their seasonal timing. In other words, spring fertilizers are advertised for spring use and fall ferts for fall use (winterizer). You do not need to fertilize in the early spring but a lot of people do. The grass will come back dark green no matter what anyway. If you fertilize early you will end up mowing 3x per week instead of 2x. The spring growth spurt will hit regardless of your fertilizer or schedule. All you have to do is wait until the grass itself slows down from the spring growth spurt. That is the timing date you are looking for. That specific date will be different every year. If you want something specific, then fertilize on Memorial Day. The same goes with the fall fert. If you want dates, fertilize once on Labor Day and again on Thanksgiving. Those dates are easy to remember.