...make it a great day!
Having a good day asks nothing of you.
It's a passive statement of mediocrity that says: Without much effort, I hope everything outside of your control meets all of your expectations so that your day can be perceived as good.
Good days require good luck and places joy outside of you.
On the other hand, telling someone to make it a great day is an active statement of empowerment and accountability.
It encourages one to show up for themselves, to live on purpose, to reframe challenges, to seek opportunities and to operate on the driver's side of life.
Making a great day comes with risk and therefore responsibility, which is why most would prefer just to have a good day.
"You cannot escape the responsibility of tomorrow by evading it today." Abraham Lincoln