Colorado income tax returns are due the fifteenth day of the fourth month after the end of your tax year, or by April 15 for traditional calendar year filers. If you are unable to file by your prescribed due date, you may file under extension. This will allow you an additional six months to file your return, or until October 15 for traditional calendar year filers. If the due date falls on a weekend or federal holiday, payment will be due the next business day. While there is an extension to file, there is not an extension to the payment due date. Penalty and interest are assessed if certain payment criteria are not met.