How can I determine what my exempt organization's tax year is? A tax year is usually 12 consecutive months. There are two kinds of tax years: Calendar Tax Year: This is a period of 12 consecutive months beginning January 1 and ending December 31; or Fiscal Tax Year: This is a period of 12 consecutive months ending on the last day of any month except December. Generally, your tax year (or accounting period) can be found in the following documents: Your organization's by-laws. Your application for federal tax-exempt status (Forms 1023, 1023-EZ, 1024, or 1024-A) or the determination letter you received approving your tax-exempt status. The application, Form SS-4, your organization filed to obtain its employer identification number (EIN). A copy of a prior year return, Form 990 or 990-EZ, that you filed with the IRS.