https://github.com/rudotriton/scriptable-calendar-widget
To keep things simple a "busy" day is defined as a day with one calendar event. This widget doesn't consider a person's work calendar, so a work day without a personal event is still considered "free".
The only exception to this rule is the "Oncall" event. The "Oncall" event is tracked but not currently used anywhere.