Members of the Communication Workers Union in England, Wales, Northern Ireland and most of Scotland 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 forms of action including a sales ban on financial products and services.
The union is opposed to plans to franchise or close 75 Crown offices, the larger sites usually found on high streets.
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.
"The company's plans are to downgrade the network, reduce services to local communities and hit jobs in the network. There is no mandate for this course of action and customers across the country are appalled at the reckless attitude of the Post Office towards these public services.
"The strike and the continuing sales ban activity will continue into the Christmas period unless management see sense and negotiate a fair deal for Crown office staff."
The union said the strike will affect up to 4,000 staff in 372 offices.
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.
"The CWU's call for its members to stop proactively introducing financial services to customers is extremely irresponsible.
"The proactive sale of our growing range of products and services is a vital part of transforming the network and meeting our objective of getting the Crown branches to break even by March 2015."