See the Javadocs in <EPM_ORACLE_HOME>/common/docs/ReportingAnalysis/SDK/javadoc.
This interface is used to define a calendar specific to the business, such as fiscal and manufacturing. CustomCalendar and CalendarYear provide a structure to define a customizable calendar. Different custom calendar types could be defined including internal, external, and default. Only one default calendar can be defined and it is constructed by the system. CustomCalendar contains a calendar year list and information about nonworking days. A calendar year contains information about a custom year, such as the period limits and quarters limits. Calendar years are defined only for internal calendars.