Usually the slip tells you to sign on a specific day, even a specific time.
Although the arrangements in different offices are different, if you've been asked to sign on during a "week commencing ..." then that would suggest to me that your office has a system which allows you to sign on whatever day suits you in the week. Local offices often pilot new schemes, I would contact them to check if this is different from your usual slip.
They brought in legislation last year to allow people to sign in by text!!! I haven't heard of them rolling this out yet though.