Members of the Communication Workers Union in most of Scotland as well as England, Wales and Northern Ireland will walk out, followed by a strike in a handful of branches in Scotland tomorrow.
It will be the 12th round of industrial action since Easter, while staff are also taking other action including a sales ban on financial products and services.
The union is opposed to plans to franchise or close 75 Crown offices.
Dave Ward, CWU deputy general secretary, said: "If the Post Office thinks this dispute will simply fade away they are sadly mistaken. Our members are fiercely opposed to the company's plans to close and franchise offices, slash jobs and impose a pay freeze."
Kevin Gilliland, network and sales director at the Post Office, said: "The CWU are calling for an above-inflation pay agreement when last year our overall performance was a loss of £116 million.
"We must reverse this loss of public money. Strike action can only cost our people money, cause disruption to customers and threaten our plans to turn the business around and keep our branches on high streets and in city centres across the UK."