Before December 31st, you need to ask your FSA provider for a grace period so that you can be given until March 15 to spend that FSA money.
That is why you need to be extra careful with your estimates before you start any FSA plan.
Remember that if you do not notify your provider about your leftover FSA money when the 12 months are over, you will not get that money back. Either you use it or you lose it if so make sure to ask for the extension.