The calendar display scrolls automatically through time when you move out of the visible portion. You can also scroll it manually. Imagine that the calendar window contains a long strip of paper with the months on it. Scrolling it means moving the strip so that new months become visible in the window.
Scroll calendar one month forward (scroll-calendar-left).
Scroll calendar one month backward (scroll-calendar-right).
Scroll calendar three months forward (scroll-calendar-left-three-months).
Scroll calendar three months backward (scroll-calendar-right-three-months).
The most basic calendar scroll commands scroll by one month at a time. This means that there are two months of overlap between the display before the command and the display after. C-x < scrolls the calendar contents one month to the left; that is, it moves the display forward in time. C-x > scrolls the contents to the right, which moves backwards in time.
The commands C-v and M-v scroll the calendar by an entire "screenful"--three months--in analogy with the usual meaning of these commands. C-v makes later dates visible and M-v makes earlier dates visible. These commands take a numeric argument as a repeat count; in particular, since C-u multiplies the next command by four, typing C-u C-v scrolls the calendar forward by a year and typing C-u M-v scrolls the calendar backward by a year.
The function keys NEXT and PRIOR are equivalent to C-v and M-v, just as they are in other modes.