It is recommended to use the joint transaction https://proaxia-prod-doc.atlassian.net/wiki/spaces/PFCC/pages/564166714 to transfer holiday calendars, holidays and assignments at once.
Use transaction /PACG/ECM_TRIG_HLDAS to transfer holiday calendar assignments from SAP ECC/S4 to SAP FSM.
Holiday calendar assignments are sent in idocs of type /PACG/ECM_ITYPE_HOLCALASS. In SAP FSM a user can be assigned to one calendar only.
Holiday calendar assignments follow the rules presented on the diagram below: