this validation rule should say "one of these three fields has changed AND Stage=Posted AND Close Date is before the current month" but it's still permitting edit of Stage, and I'm not sure the date piece is quite right. Can you help, and if so, can you also tell me what my formula is saying instead of the above, so I can learn?
AND(
OR (
ISNEW(),
ISCHANGED( CloseDate ),
ISCHANGED( StageName ),
ISCHANGED( Region__c )),
CloseDate < DATE( YEAR(TODAY()),
MONTH(TODAY()), 1),
ISPICKVAL(StageName,"Posted"))