It is to highlight just how much of the working year is worked as unpaid overtime by the millions of workers routinely working beyond their additional hours for no extra pay.
Unfortunately, the very title of the day suggests that the aspiration of our movement is that there should only be one day a year when our members don't put in unpaid overtime! (I realise that isn't the intention of course).
In the mid nineteenth century the nascent trade union movement fought a lengthy struggle to limit working time. It was the success of this struggle which drove employers to seek increased profits not from simply lengthening working time, but by revolutionising production processes to increase the productivity of each working hour.
The struggle to work less was therefore the foundation for the massive increases in labour productivity which have underpinned rising living standards over recent generations.
It's no accident that in the past generation, in which the increase in the living standards of working people has been first stalled and then reversed, we have seen a tendency for working hours to increase, often without pay.
When I started work in local government in the 1980s, to have worked more than your contracted hours would have singled you out as, at best, an oddball and, at worst, someone seeking to curry favour with management.
As we approach once more three million on the dole, we need to recreate the consciousness that those who work overtime are stealing work from our unemployed brothers and sisters (those who work it without pay are doing so for nothing).
If you want to do more than your 35 (or 36 or 37) hours - volunteer to do some recruitment work for the union. Our message to our members should be "work your proper hours every day" - and perhaps we should demand that employers refuse to permit unpaid overtime and (if they won't) ballot trade unionists in every industry for an indefinite overtime ban?
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