Make sure the flag is set to post in the 13th Period in File/Preferences/Settings.
The date must be entered on EVERY TRANSACTION you want to post in the 13th Period. You must always TAB out of the date field. For each transaction where you enter the date as the last day of the fiscal year, when the flag is set, you will be prompted as shown below. Answer ‘Yes’ for those line items you want to post in the 13th Period.