An interesting feature in Salesforce. Important things to note,

1. Standard Fiscal Year – Follows the Gregorian calendar year, but starts on the first day of any month you choose
2. Custom Fiscal Year – Follows a custom structure you define. For example, you could have 4 quarters with 13 weeks per quarter in a 4-4-5 pattern or 13 periods per year.
3. Custom fiscal year once enabled to an Org, cant is deleted. So if we need to bring it back to 12 months period then we can change and mimic it like the standard calendar year.
4. A custom fiscal year can have any number of quarters and each quarter can vary in a number of weeks.
5. Enabling or defining custom fiscal years impacts your forecasts, reports, and quotas. When you define the first custom fiscal year, all existing forecasts, forecast history, and forecast adjustments from the year’s first period forward are deleted