States, schools, and the federal government have their own financial aid deadlines, so you should apply by the first relevant deadline for you. You can find state deadlines here.
Also, note that financial aid can be limited and offered on a first-come, first-served basis, so it’s best to submit the FAFSA® form as soon as possible after it opens (previously, the FAFSA® opened on October 1. If you’re applying for aid for attending college in 2024-2025, the FAFSA® will open in December 2023). The sooner you complete the FAFSA®, the more aid you could be eligible for.
But don’t stress out if you’re late. The FAFSA® gives you a full 18 months to file. That means students who start college in the fall of 2024 can start filing in December 2023 but may still file the FAFSA® as late as June 30, 2025. In that case, students just receive federal grants or loans retroactively.