What word or phrase is used to signify that the work day has ended?
If you call it a day, it means that you consider the day's work is done.
They may say that they are going to "call it quits":
to quit; to resign from something; to announce that one is quitting.
Okay! I've had enough! I'm calling it quits.
Time to go home, John. Let's call it quits.
In this sense, quit is not used to mean that they are leaving their job for good but that they are leaving for the night. A related phrase is "quitting time":
Ok, 5pm. It's quitting time. I'm going home.
The simplest thing is:
It's correct to say
optionally with "See ya tomorrow (or next week)"
When the day has ended, you usually say "It's nighttime" :)
When you are stopping your work, you say "I'm calling it a day"
"It's Miller Time!" in reference to old advertisements for Miller beer.
"One last call for alcohol ... you don't have to go home, but you can't stay here."
How about "knocking-off time"? Collins English Dictionary