Postmen and women across the country will wear festive gear on Monday to mark the first-ever national postal workers day.
The Communication Workers Union is organising the celebration as part of its campaign against privatisation.
Staff will don Santa hats in towns and cities and help publicise the union's warnings about the effect on services under the Government's controversial plan to sell off the Royal Mail.
Billy Hayes, CWU general secretary, said: "In many communities people are reassured that a local postwoman or man, who everybody knows, is a point of daily contact who provides a lifeline for the elderly and isolated.
"Our experience is that private postal companies don't pay a living wage and put workers on zero hours contracts which means they never know when or where they are working.
"It destroys family life and takes the postmen and women out of the communities where they have been embedded for years."
Over 100 million items are expected to be in the Royal Mail system tomorrow, one of the busiest days of the year.
Dave Ward, CWU deputy general secretary, said: "Six days a week, 52 weeks a year, in heat waves, floods, and blizzards the UK's postal workers are out on their walks, on the road, and in mail centres playing their part in ensuring that this vital public service delivers to everyone, everywhere.
"We are asking people to help us celebrate the great work of postal workers and support our campaign to protect the universal service , which guarantees the same price for mail to anywhere in the UK.
"On December 17 say thanks to your postwoman or man, and express your support for the amazing service they provide."