Validation Rule:
AND(
IF( IsNew(), true, IF( PRIORVALUE( IsClosed ), ISCHANGED( CloseDate ) , false ) ),
MONTH( CloseDate ) != MONTH( TODAY() ),
IsClosed
)
Opportunity Closed Date must be in current month Salesforce Validation Rule
Validation Rule:
AND(
IF( IsNew(), true, IF( PRIORVALUE( IsClosed ), ISCHANGED( CloseDate ) , false ) ),
MONTH( CloseDate ) != MONTH( TODAY() ),
IsClosed
)