"Let's call it a day" is an idiom, even a cliché. Meaning "Let us decide to end work now".
It is productive enough that "Call it a week" is a natural development. People normally have a break from work at the weekend, analogous to the break from work overnight. It and has plenty of examples on the internet. It would be less natural to say "Call it a month" because normally people don't have a break from work at the end of the month.
You don't need any sentence to end the day or the week. If you say the same thing everyday it would sound boring. There are so many things you could say "I'm off now", "Have a great weekend, everyone", "See you all on Monday" and so on.