Members of the Communication Workers Union and Unite will walk out on one of the busiest days of the year, mounting picket lines across the country.
The unions are embroiled in a long-running row over the franchising of Crown Post Offices - the larger branches usually sited on high streets - plus job-losses and the closure of a final salary pension scheme.
Dave Ward, general secretary of the CWU, said: “The Post Office is now at crisis point. It needs a new strategy as a matter of urgency.”
Unite union officer Brian Scott said: “We believe that Saturday is the day when most people will be despatching their cards and parcels to their relatives and friends abroad.
“We are taking this action because the management refuses to talk in a constructive manner about the pension scheme which is currently in surplus to more than £143 million.
“This is the retirement income of our members which is at stake and we are not going to stand idly by and let them lose thousands of pounds when they retire.
“More generally, it appears that it is only the unions that care about the future viability of the Post Office and the services it provides for communities across the UK.
“The management seems to have abdicated its responsibility and, as the Government ultimately owns the Post Office, we call, once again, for Business Minister Margot James to order an investigation into the Post Office’s future and what we consider is a catalogue of managerial incompetence.”
The Post Office told customers that “almost all” of its network of 11,600 branches will be open despite the strike.
Kevin Gilliland, Post Office network and sales director, said: “We want to reassure people that almost all of our network will be open and operating normally.
“Action affects fewer than 300 of our branches and many of these will be open to maintain services for customers preparing for Christmas.
“It will also be business as usual in the rest of our network - over 97% of branches will be operating their usual Saturday service.
“Instead of trying to unnecessarily alarm customers during the festive season, we want the unions to join us in talks to discuss our strategy.
“Their members work very hard to give our customers the best possible service in the run-up to Christmas. This action can only cause concern to customers.”