This is set in Accounts>Nominal>Utilities>Set Up>Back Posting Prevention.
This prevents anyone posting to a nominal date before the one displayed. This date should therefore be the first of the new month and not the last day of the previous month.
Once this is set, a user will require an override code to be able to post to a date prior to the one that is set.
This code is set in Set Up> General Set Up>Nominal Back Posting