Fiscal years and the periods entries post into.
Periods grant permission to post
Every entry has to land in a period that is open. A closed or locked period accepts nothing.
So "the entry will not post" with a correct date almost always means that date's period is not open.
Closing a year is a real entry
Closing a fiscal year is not only a lock: it writes a closing entry that moves revenue and expense balances into retained earnings, and then locks the year.
It affects the books and appears in the history like any other entry.
The balance sheet balances before you close
You do not have to close a year for the balance sheet to be right. An open year's profit is shown as a computed line in equity, so it balances at any moment.
A fiscal year's calendar is its own
Each fiscal year carries its own calendar system, and that is what its month boundaries use. A Jalali fiscal year on a Gregorian site is possible — its dates are shown in its own calendar.
A year with entries cannot be deleted
A new fiscal year with no entries can be removed. As soon as anything posts into it, deletion is refused.