In the UK, it’s probably illegal to make you work on a Sunday:
If [your employment contract] doesn’t [specifically say “you must work on Sundays”], then the only way of making you work on that day is by a change to your contract. This is something that must normally be agreed by both you and your employer, otherwise making you work on Sundays would amount to a breach of contract.
And even if your contract is paying you extra to work on Sundays, it’s still illegal to make you work both the Saturday and the Sunday back-to-back (modulo some very specific exceptions which are almost impossible in the games industry):
[and] you have the right to either:
* an uninterrupted 24 hours clear of work each week
* an uninterrupted 48 hours clear each fortnight
If you work for a UK games company, and you’re working more than 5 days a week (you’re “crunching”), send your manager a link to this page. You don’t need to threaten to sue them – they’re breaking the law already, and they know it, and they know perfectly well they’re screwed (by themselves).
Just gently point out that you hope your manager will fix it before someone reports it *their* manager – wouldn’t they rather look good (“I noticed this problem before we got sued”), than be the one to take the blame?
There’s no excuse for this kind of behaviour. Don’t let them do it to you.