Applying for fellowships is an extended process. Putting together a strong application is equivalent to taking on a 2- to 3-unit course over a quarter, and you want to give your letter writers more than a month’s notice.
Many deadlines fall in late summer or autumn. Students planning to apply in summer/autumn will want to look out for application workshops in the previous spring. The earlier you identify the fellowships that interest you, and note the timeline for preparation, the better off you’ll be.