In this lesson, I show you where to set an organization’s fiscal year setting. By default, an org’s fiscal year is set to the default fiscal start of January. We walk through the process of how to set a Fiscal Year to a Custom one, but we don’t save our changes.
I also discuss some of the warnings and impacts that adjusting a Fiscal Year in Salesforce will have on your org and especially on your Forecasting.
Define Your Fiscal Year - https://help.salesforce.com/articleView?err=1&id=admin_about_cfy.htm&type=5