By default, Planboard rotations are labeled with letters (Day A, Day B, etc.), as shown below.
You can control whether the dates in your rotation are annotated with letters (Day A, Day B, etc.) or number (Day 1, Day 2, etc.) through the Settings menu option on the left side menu. You'll find your settings in the top left corner by clicking the three horizontal lines.
Scroll down the navigation menu to find your Settings.
Once in Settings you can toggle off the option to use Letters instead Numbers as seen below.