It is an organizational category where you put any action that you need to get done at a specific time or on a specific day.

Actually, there are three kind of stuff that will end up on your calendar:

1. Actions that you need to perform at a specific time.
2. Actions that you need to perform on a specific day.
3. Information that you need to take into account on a specific day.

In GTD, the calendar should be sacred territory. If you write something there, it must get done that day or never. The only rewriting should be for changed appointments.

By doing this, you can trust that your calendar always reflects the exact hard edges of your daily commitments.