Resource:
Cumulative Forecast Rollups in Collaborative Forecasts - https://help.salesforce.com/articleView?id=forecasts3_cumulative_columns_overview.htm&type=5
Resource:
Customize the Fiscal Year Structure - https://help.salesforce.com/articleView?err=1&id=admin_cfy_customize.htm&type=5
Define Your Fiscal Year - https://help.salesforce.com/articleView?err=1&id=admin_about_cfy.htm&type=5