The calendar in NaturalHR is really useful for users to view their own, and possibly other people in the business's time off, birthday, work anniversaries and national holidays. If you have the Natural HR Training module enabled then you will also see those entries too.

The colours that appear on your calendar can be customised by navigating to Admin > Options > Calendar options 

Click here to view the video to see how you can choose the colours that appear on your calendars, and, if enabled, the Planner feature as well.