Is the deadline for contributing to a Roth IRA? Dec. 31 or when I file my taxes?
According to the IRS page on Roth IRAs:
You can make contributions to a Roth IRA for a year at any time during the year or by the due date of your return for that year (not including extensions).
So, April 15th is your deadline (regardless of when you file, particularly since Roth IRA contributions are not deducted and thus not listed on your return). Extensions don't advance this.
This question addresses the possibility of contributing after your return is complete; it suggests one good strategy, which is to take your refund and contribute it to a Roth IRA for the previous year (which is fine as long as you're under income limits and get it in time).