We do not recommend using period 13 in your General Ledger Calendar.
Foundations will occasionally use period 13 for audit adjustments or for major re-allocations of fund balances, but the actual assignment of period 13 will occur just prior to posting the journal entries and as soon as the entries were posted, the calendar will be changed back to a normal 12 period year.
Because the actual period used in posting a journal is not recorded on the journal entry, it is impossible to systemically identify which journals posted to period 13 and which posted to period 12.
For that reason, use batch codes and/or reference numbers when posting to a period 13 so the journals can be identified after posting.
To create period 13, run File Maintenance>General Ledger>GL Calendar Maintenance and remove the last day of period 12 and move it to period 13, save and close. After posting the period 13 journal entries, reverse the procedure.
Keeping a calendar with period 13 associated with the last day of the year is risky because normal accounting events might be dated on that date and usually these would not be true period 13 entries.