The union claims it is taking the action in a bid to force Royal Mail into "meaningful talks over modernisation" because "pressure and stress is at breaking point for postal workers".
Royal Mail responded by condemning the union's tactics and claiming that more than 90% of its people would still be delivering the mail to their customers with over 90% of offices not affected by the planned strikes.
Other offices not on strike will play a role in the action by joining in a national release of thousands of balloons.
This will follow the delivery of a letter and postcard to Royal Mail's chief executive Adam Crozier and business secretary Lord Mandelson.
Dave Ward, deputy general secretary of the CWU, said: "Royal Mail has abandoned the final phase of the 2007 Pay and Modernisation agreement which mandated the company and CWU to negotiating modernisation."
The CWU ran a three-day strike in London from last Friday, July 10.