Workers put in £23 billion of unpaid overtime last year, losing out on an average of £4,800 each, according to a study highlighting the UK's continuing long-hours culture.
The TUC said people working for nothing were doing an average of seven hours and six minutes extra work every week.
The union organisation calculated that if employees did all their unpaid work at the start of the year, the first day they would be paid would be February 23.
People were urged to work their proper hours on February 23, take a decent lunch break and go home on time.
TUC general secretary Brendan Barber said: "We still work the longest hours in Europe and too many workplaces are gripped by a long hours culture.
"There are some small signs that the UK is getting a bit better, but there is still a long way to go.
"That is why we say that employees should take a stand on 'Work Your Proper Hours Day' and for just one day a year take a full lunch break and go home on time.
"We do not want to turn Britain into a nation of clock watchers and few mind putting in extra effort from time to time when it is needed, but it is too easy for extra time to get taken for granted and then expected every week."
Workers in the North East did the most unpaid overtime last year at an average of seven hours 42 minutes a week, followed by those in London (seven hours 36 minutes), Eastern England, the West Midlands, Yorkshire and the Humber and Northern Ireland (all seven hours 12 minutes).
Workers in Scotland did the least amount of unpaid overtime at six hours 30 minutes on average.
