The TUC said the fall in wages ranged from £1,300 for police officers to £3,300 for midwives. Nurses pay was down by £2,600 compared to five years ago.
Around 900,000 people in total work on Christmas Day, the TUC estimated.
General secretary Frances O'Grady said: "For most of us, Christmas is a well-earned break from the daily grind. But nearly a million people will be working on Christmas Day this year.
"Many of those keeping our streets safe and providing emergency care have seen their pay fall sharply over the past five years.
"Ministers should show some seasonal goodwill and end the real-terms pay cuts. The Government's Scrooge-like public sector pay cap has to go, to ensure that wages at least keep up with prices."
Unison general secretary Dave Prentis commented: "Every year emergency service workers put their lives on hold so the rest of us can enjoy the festive season.
"Yet they continue to be paid well below their worth by a Government that sees the cost of everything and the value of nothing.
"A decent pay rise this year would be a great Christmas present for selfless staff who ministers have taken for granted for far too long."
A Treasury spokesman said: "The Government has made difficult decisions on public sector pay to maintain fiscal discipline and protect public sector jobs.
"The OBR has forecast that current pay policy will protect approximately 200,000 jobs across the UK."