weekbeginning
Saturday is the weekend, and Sunday is the weekbeginning.

If Sunday is the first day of the week, why do we say it is part of the weekend?

If we have a closed compound, "weekend", why don't we have a closed compound, "weekbeginning"?

When you say goodbye to your coworkers on Friday afternoon you should say, "Have a nice weekend, and a good weekbeginning."
It's 5:00 on Friday afternoon and there is a cacophony in the office of coworkers bidding each other to have a nice weekend and a good weekbeginning.