The sentence "24 hours a day, seven days a week" is correct and can be used in written English. You can use it to indicate that something is available or accessible at all times, such as when expressing availability of a service. For example, "Our customer service staff are here to help you 24 hours a day, seven days a week.".
They work 24 hours a day, seven days a week.
The pier is open 24 hours a day, seven days a week.
"It just looked exciting, open 24 hours a day, seven days a week," she said.
It goes on 24 hours a day, seven days a week.
He is on call 24 hours a day, seven days a week.
Pusher Street is open 24 hours a day, seven days a week.
There were people outside her home 24 hours a day, seven days a week.
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