It would help keyboard users to navigate faster within the open calendar, if there are keys that would switch years directly. In that case, focus would move to the same day in the previous/next year.

The keys should be as follows:

CTRL-PAGE_UP - Move focus to the same day in the previous year.
CTRL-PAGE_DOWN - Move focus to the same day in the next year.

On Mac, please use CMD over CTRL instead.