Is it bad to use new MacBook Pro with battery every day? Or would it be better for the battery lifetime to use battery only one day a week?
The problem is that I don't think I can change battery if it gets bad...it's built into the MacBook I think.
There is no harm in running on battery most of the time.
To get the most out of your MacBook Pro's battery, follow the Notebook Battery advice from Apple: unplug and use your battery until empty about once a month, then charge back up to full.
If you need help following Apple's advice, use Battery Guardian; it is free and will remind you when to deplete your battery.