How do I change the start of my workweek or workday?

You can configure when your workweek and workday begin from your Company Settings.

Click your profile in the bottom-left corner and select Company Settings, or use the search bar at the top left (or press Ctrl+K) to look for it.
Go to the Timekeeping section.
Set the day you want your workweek to start.
Set the time you want your workday to start. By default this is set to midnight, giving you a standard 24-hour day. For example, if you set it to 6 a.m., your workday will run from 6 a.m. to 6 a.m. the following day.
Save changes.