If you are trying to change your fiscal year from one ending date to a different ending date, this can't be done by simply editing Fiscal Calendar, as it requires a full year when validated.

Here is a link to another forum posting that describes having a short fiscal year when the desire is to change to a different year end date:

Change Ledger year end date

There are a number of other threads that I found by using the Search function on this forum and using the criteria '+change +fiscal' - perhaps other threads may be of interest to you.