The earliest you can file the FAFSA is January 1st of senior year in high school, so you will want to file soon as possible after that date. The PROFILE can be sent in at any time, so filing early – before January 1st – is advisable.
If you anticipate qualifying for need-based financial aid, you must file no later than March 1st. Many states and most colleges use the March 1st filing deadline to close eligibility for state and campus-based aid. By missing that date, you may be ineligible for those kinds of aid, your need notwithstanding. Do not miss this deadline.