The strikes organised by the CWU are part of a five-month dispute over closures, job cuts and pay.
The long-running row is centred around the Post Office's plan to franchise or close more than 70 Crown sites, (usually the larger branches found on high streets).
CWU deputy general secretary Dave Ward said Crown Post office workers do not agree with the management's "slash and burn" approach and said it's time to resolve the situation.
"We are prepared to take prolonged industrial action to defend jobs and services and win a fair pay rise," said Ward.
The CWU said strike action would affect up to 4,000 staff working in 372 Crown (main) post offices.
Post Office staff voted by nine to one (88%) in favour of strike action and employees have already taken strike action on nine previous occasions over this issue.
