You can manage which periods are open and when through the use of the Period Status. The periods are created with the Status of Inactive, "I". When a period is to be used for input the Status is set to Active, "A", using the Open a Period function. Journal Entry will check for a valid status, and check transaction dates for reasonability. Releases from subledgers will also check for a valid status and a reasonable end date. When you are done with a period and want to restrict input to it, set the Status to Not Open, "N", using the Close a Period function.
Month-end, or period close, can be an arbitrary event in the General Ledger system. This happens when all the periods are changed to a Status of Active, "A". The accountant then decides when a period is complete and no more data is to be posted to it. At this time final Trial Balance and Detail of Account reports should be printed for the period. If adjustments are made to the period at a later date the final reports for the period should be reprinted.
Some organizations, like Louisiana Pacific Canada, have functions that are usually done at the end of each period. These appear on the Monthend Processing menu which is customized for the company.