Royal Mail workers, university lecturers and sixth-form college staff will walk out over pay disputes today as strikes continue across the country.
Picket lines will be mounted outside universities, colleges and Royal Mail centres in one of the biggest walkouts on the same day.
Royal Mail has warned that the postal strikes will "hold Christmas to ransom" for their customers.
Members of the Communication Workers Union (CWU), which represents Royal Mail staff, are also planning strikes next month, including on Christmas Eve.
Dave Ward, its general secretary, said: "Royal Mail bosses are risking a Christmas meltdown because of their stubborn refusal to treat their employees with respect."
Postal workers want to get on with delivering Christmas gifts, he said, but "they will not meekly accept the casualisation of their jobs".
Hundreds of thousands of workers across many sections of the economy, including nurses, rail workers and ambulance staff, are due to strike next month and in January.
Rishi Sunak, the prime minister, has warned the winter will be "challenging" as services stop over pay disputes.
The army is on stand by in case it is needed to fill roles during strikes by NHS workers over the winter.
"The reality is if the army or other armed forces step in it will very much be at the margins rather than going out and driving ambulances," Saffron Cordery, the interim chief executive of NHS Providers, told Sky News this morning.
The CWU said its members will be in London next Friday for the "biggest strike demonstration this country has ever seen".
Royal Mail said it was "proud to have the best pay and conditions in our industry" and has urged customers to send their Christmas post early.
National Education Union (NEU) teacher members who work in 77 sixth-form colleges in England are also on strike in a dispute over pay.
The union said teachers working in sixth-form colleges have suffered a real-terms pay cut of an estimated 20% since 2010.
A ballot showed huge support for industrial action.
Dr Mary Bousted, joint general secretary of the NEU, said: "Members take strike action with great reluctance, but the effects of real-terms pay losses are simply too urgent for them to endure the situation any longer.
"These cuts are driving an exodus from the profession."
The University and College Union (UCU) is following a 48-hour strike last week with a 24-hour walkout among university staff and is holding a rally in London.
UCU members at 150 universities will be on strike.
Members at the University of Sheffield International College are on strike for three days, ending on Wednesday, in a long-running dispute over low pay.
The union says the action is the first-ever strike to take place in a privatised higher education provider.
Rail workers are due to strike for eight days next month and in January, the RMT union announced last week.
Mick Lynch, its general secretary, held talks with Mark Harper, the transport secretary, last week but there has been no breakthrough in the long-running dispute over jobs, pay and conditions.