About 3,000 staff at hundreds of Crown post offices are expected to walk out on Monday, Tuesday and Saturday, the BBC reports.
But the Post Office said fewer than 300 branches would be affected, with “business as usual” in most places.
Meanwhile the rail strike at Southern continues with conductors beginning two days of action.
Talks aimed at averting a strike by British Airways cabin crew over Christmas are also to be held on Monday.
Are we facing a Christmas of Discontent?
Workers at Crown post offices, the larger branches usually found on the High Street, are protesting against pension changes, job security and closures.
On Wednesday and Thursday, workers who supply many sub-post offices with cash will join the action.
There are fears the situation could escalate if unofficial action is taken by Royal Mail workers – who are not currently involved in the dispute – and they refuse to cross picket lines.
A Royal Mail spokesman said: “There will be little or no impact on Royal Mail as a result of the CWU strike at the Post Office. Deliveries will carry on as normal and the last posting dates for Christmas remain unchanged.”
Post Office communications officer Mark Davies told the BBC that the last time such action was taken only 80 branches were affected.
He said that the company was trying to become more efficient and reduce its losses, which would be good news for the taxpayer.
“We have reduced losses from £120m four years ago, to £26m in the last year, and we hope to break even next year. We don’t believe that taxpayers should pay out to support the Post Office, ” he said.
Meanwhile, guards at Southern rail will strike on Monday and Tuesday in a dispute over the role of conductors.
Along with a continuing ban on overtime by drivers in Aslef, the action will cause more misery for Southern’s 300,000 passengers.
RMT general secretary Mick Cash said: “Our conductor members on Southern are on strike this week in defence of the safety of the travelling public and that remains the focus of the entire union.”
Southern was accused of cancelling trains and blaming industrial action even though drivers were available, for the second consecutive weekend. The company has denied Aslef’s claim.
British Airways cabin crew belonging to the Unite union are due to strike on Christmas Day and Boxing Day in a row over pay.
Talks aimed at averting the strike are due to be held on Monday at the conciliation service Acas.
In addition, Unite members employed by Swissport as baggage handlers and other ground staff at 18 airports across the UK are set to walk out on Friday and Saturday over pay, although talks will also be held at Acas on Tuesday.
Members of the Transport Salaried Staffs Association and RMT on London Underground are continuing with an overtime ban in a dispute over jobs and ticket office closures, while pilots at Virgin Atlantic will start a “work to contract” action from Friday in a row over union recognition.
Prof Roger Seifurt from the University of Wolverhampton said many of the tensions were longstanding and were “boiling over now because during Christmas they get more media coverage”.
He told the BBC’s Today programme that discontent among workers was not just affecting these companies, and that worsening pensions, stagnant pay and reduced job security was hitting a range of industries.
“This is now boiling over into strikes which I think will carry on into the new year.”
He added that we were entering “a period of quite bitter and prolonged disputes” where unions will take more strike action.
Frances O’Grady, general secretary of the TUC trade union body, pointed out that all the strikes were for different reasons and that the focus should be on the root causes of the problem.
“We are working in a background where workers are still £20 a week worse off on average than before the financial crash, where we’re seeing inflation rise and real pay cuts come in. We’ve had the rise of zero hours and sham self employment.
“Unions are membership organisations of ordinary working people, and striking is always a last resort, We negotiate and settle thousands of deals.”
Ms O’Grady added that she feared there might be “another agenda” here to tighten union laws.
Conservative MP Chris Philp is calling for new laws that would mean in cases involving critical public infrastructure, such as the railways, a High Court judge would decide whether a strike is “reasonable and proportionate”.
He also wants similar legislation to that in Spain, Italy and Canada, which means that when there is a strike, 50 per cent of the service still runs.