Filling holes in your schedule before filling an empty day is usually preferred; but you get to adjust how much Gazelle favors 'Open Days' vs. 'Days you are already on the road' is easy! (NOTE: Any day you are available for scheduling where you currently have nothing scheduled is called an "Open Day").
FROM THE WEB APP
Step 1: Go to Scheduling > Scheduling Policies > My Policies
Step 2: Edit your "Open Day Policy"
Step 3: Save.
FROM THE MOBILE APP
This setting can only be changed by logging into the Web App at this time.