Your deadline to enroll in a FSA is dependent on what type of enrollment is occurring.
New Company FSA Plan or FSA Open Enrollment
When your company sets up a new company FSA plan (initial FSA enrollment) or renews an existing plan (open FSA enrollment), you have until the 25th of the same month to set up your individual FSA to start on the 1st of the following month.
New Hire FSA Enrollment
When you get hired at a company with an existing FSA plan, you'll need to complete enrollment for your FSA within 30 days from your hire date. Your FSA start date will depend on your hire date. If you do not meet this deadline, you will need to wait until your company's next FSA renewal period or enroll if you experience a Qualifying Life Event.