With an August 31st year end when you move the Session Date to September 1st Sage 50 will complete all of the year end processing in the program such as closing all revenue and expense accounts to the retained earnings. Retained earnings will be moved to Retained Earnings - Prior Periods. Adjustments to the Balance Sheet accounts will have to be completed manually.

At this point you will still be able to make entries in the prior year to clean up late arriving invoices etc. just by using a date in the prior period. When the accountant has completed their work, their adjusting entries can still be made in the prior period. All of these entries will be properly reflected in the August 31st financial records and the opening balances will be properly adjust to start the new year.

As soon as you have moved the Session date to the new year you will be able to entering information for the new year without concern for the prior year entries still to be made.

Jaquila said:

Should I have something come in with a September 1st date or later before I have processed the fiscal year end, am I able to post that transaction with a date in the new fiscal year?

If you are using a version from 2013 or higher, then you can post General Journal and payroll entries into the new fiscal year before you move the session date to the new fiscal year.  No other entries can be posted past Fiscal year end.