Changing My Calendar

So, I’ve used Monday Hour One to plan my week from commitments to social events to focus time at work to housework, errands, etc. Now it’s Wednesday and I think I want to run errands after work tonight instead of tomorrow as planned, or I have an extra 30 minutes because I overestimated when I’d get out of work so I want to run errands today. I tell myself it’s ok to change the calendar but then feel guilty because I’m not following it to a T based on what I planned on Monday. Should my calendar constantly be changing or should I view it as set in stone and honor what I planned?

Should I be scheduling every minute of the day for something even if it’s “downtime/me time” or leave some blanks for overages for things that take longer than originally planned?