Notice is sometimes part of a CF agreement. It is pretty much just an announcement to end a cease fire between 2 kingdoms.
When KD A and KD B agree to CF until January 1st YR2 + 72h notice, either of these kingdoms may give the other kingdom their notice on or after January 1st YR2. If KD A notices KD B on January 1st YR2, their cease fire may be broken as of April 1st YR2.