Most individual tax returns cover a calendar year, the 12 months from January 1 through December 31. If you do not use a calendar year, your accounting period is a fiscal year. A regular fiscal year is a 12-month period that ends on the last day of any month except December. A 52-53-week fiscal year varies from 52 to 53 weeks and always ends on the same day of the week. You choose your accounting period (tax year) when you file your first income tax return. It cannot be longer than 12 months.

This information is found in the Filing Information chapter of Publication 17, Your Federal Income Tax.