It depends on your income sources. I would suggest you do your 2015 taxes first, before you even start your 2016 return. This can really matter if you have things like a business, rental property, carry over losses, etc.

Remember, you can not e-file your 2015 return. You will have to print it, sign it, and mail it to the IRS. If you try to e-file it, you'll be e-filing it as a 2016 return, and that will mess you up royally. So don't even try.