What are the three-minute and six-minute calendars of watch movements?

There is a calendar display window at 3 o'clock on the dial.

Or there is a calendar window at 6 o'clock.

It's not that hard to understand. Literally.

Just give you a picture.

As shown below, the calendar window at 3 o'clock position.

And below, the calendar window at 6 o'clock.