What is the difference between resource calendars and task/project calendars?

The task/project calendar determines on which days the task/project cannot be planned.

For instance, meetings cannot be planned on Monday, so you indicate this constraint in the task calendar and assign this calendar to the meeting activity.

The resource calendar has to be used to define the availability/unavailability of your resources.

For instance, John is available at 50% on Friday, so you set it up in John’s calendar.