Accounting periods
Under Finance → Accounting periods you close each month once its figures are complete, so nothing more posts into it.
This article is still being written: it explains what the screen is for, and its steps are not written yet.
When a month's figures are complete, you close it on this page. A closed month accepts no posting: a late document posts in the open period, or the month has to be reopened. Before you close, the page points out what isn't finished yet, such as a document that failed to post or the month's payroll that hasn't been posted.
You'll find it in the sidebar under Finance → Accounting periods. Owners and accountants can open it, and so can anyone given a role with the permission to close or reopen periods.