What works for us is to set expectations far ahead of time.
in the evening:
Tomorrow is a work day, daddy will leave for work before/after you have had breakfast (or whenever). He will be back tomorrow night by dinner time.
In the morning:
I will leave soon/after breakfast, today is a work day. (repeat, countdown). I will be back by dinner time.
REPEAT, REPEAT, REPEAT. Set expectations.
Point out differences when not working:
Tomorrow is not a workday, I am not going to work. I am going to be home all day/I am going to run errands in after lunch/etc
Again, set expectations, and do not think because they understood yesterday means they understand today.
If someone has to leave for business travel, we talk about it days ahead. Every day we repeat that mommy/daddy is not home tonight, and when they will be home.
Of course it is important to try and do your best to live up to these expectations.