The Communication Workers Union said its members will walk out in the latest wave of industrial action linked to plans to franchise or close more than 70 Crown sites - the larger branches usually found on high streets.
A UK-wide stoppage will be held on August 24, staff in Scotland will strike on August 26 and union members in England, Wales and Northern Ireland will stop work on the 27th. The five-month dispute is over closures, job cuts and pay.
Dave Ward, CWU deputy general secretary, said: "Workers do not agree with management's slash and burn approach and are prepared to take prolonged industrial action to defend jobs and services and win a fair pay rise."
Post office workers yesterday voted to take industrial action short of strike. Notice is likely to be given next week for a ban on selling financial products.
Kevin Gilliland, network and sales director at the Post Office said: "This action can only cause disruption to customers, cost our people money and place further pressure on the Crown network which is currently losing £37 million a year.