You must be hard core if you clicked to come to this page. Tax accounting is based on the concept of the tax year. You could have big swings in taxable income from year to year, but that doesn't matter. The IRS looks at one year at a time.
Most small businesses use a calendar year just like individuals.
Fiscal years can be used to manipulate tax, so the IRS sets up the calendar year as the default requirement. You need a good business reason to pick another tax year. But many businesses, particularly seasonal businesses, do use a fiscal year. For example, a retailer who does his main business at Christmas may pick a tax year that ends on January 31 of every year. For this retailer, it doesn't make sense to do a closing inventory on December 31.