About 720 managers will walk out of Post Offices over pension disputes on the busiest day for international post.
The 24-hour strike, set for Saturday, 3 December, is the third instance of industrial action in a long-running pension dispute.
The workers are members of Unite, the country's largest union, who join the Communication Workers Union (CWU) to target 300 locations.
"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 £143m," Brian Scott, Unite officer for the Post Office, said in a letter to the managers.
The day was chosen because the union believed it was the peak day for people sending cards and parcels out to relatives and friends abroad, Scott said.
Read more: Post Office workers set to strike next week
The union said management's has refused to reconsider the closure of the defined salary pension scheme at the end of March 2017.
Unite is also concerned about "the lack of a future coherent business strategy by the top bosses and supports the CWU call for a Postbank to be established at the Post Office", according to a report.
In the letter the union called for junior business minister Margot James to order an investigation into the Post Office's "catalogue of managerial incompetence".
The Post Office managers voted by 64 per cent for strike action, with 78 per cent supporting industrial action short of a strike. There are 11,500 post offices across the UK.
Unite calculated that under the defined contribution scheme from next April, based on the vagaries of the stock market, staff will lose about 30 per cent of their retirement income going forward – thousands of pounds a year. The defined contribution scheme is already in operation for other members of the Post Office’s 7,000-strong workforce.