Financial year variant periods & incomplete (31.12 missing)

What causes this issue?

A complete financial year variant definition contains entries for every
day in the calendar year, i.e. it must be possible to find the posting
period for every calendar day.
This includes an entry for the last day of the year, which is missing
here.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Specify a posting period for the last day of the year.

Error message extract from SAP system. Copyright SAP SE.