"Call it a day!" is more common. It means that you may consider the day (or week) complete. All the necessary tasks for the day (or week) are finished. CJ
Let's call it a day/night/week all mean "let's wrap it up," or "that's all we're gonna do today/tonight. That's all there is for this week."
Perhaps the child is away at school and typically comes home to visit on the weekend. Or perhaps the parents are divorced and have a shared custody arrangement, where the mother's share of weekly custody has just been completed.