I applied to an Ivy and got deferred, but I submitted the financial aid application along with the regular app. I'm still interested, and I've sent a letter of continued interest, but I'm wondering if I need to submit FAFSA and CSS now in order to get financial aid.
Yes, you need to submit your FAFSA and CSS to get financial aid, even if you've been deferred. The deadline by which you have to submit this information varies from school to school, and may be different if you are an international student or applying from outside the US. Most of the Ivy League schools (Cornell, Columbia, UPenn, Harvard, Dartmouth, and Yale) require CSS information be submitted at or near the November early decision/early application deadlines, while Georgetown and Princeton have slightly different timelines. For all the Ivy League schools, though (and indeed, for most colleges), you still have to also submit the FAFSA to get financial support.
You'll need to check the websites for your specific school to get specific deadlines, but in general FAFSA deadlines are usually early April/May (and the earlier you submit, the better). I've linked to the eight Ivy League schools' respective financial aid pages here, so you can double check the deadline for your school:
Remember, FAFSA is not only required for federal student aid, but also is often required for state- and college-based aid. It's better to submit your FAFSA to a school you've been deferred to and later find out you've been rejected by the school than to get into the school, only to realize you can't afford to attend it because you neglected to submit your FAFSA and so can't get federal student aid. The only exception to this situation would be if the school you're applying to doesn't have needs-blind admission; however, since you already submitted a financial aid application with your initial application to the school, they're already well aware that you're seeking financial aid, and neglecting to submit the FAFSA at this point can only hurt your chances of getting aid.
For a simple explanation of what exactly the FAFSA is, read our article!