A Landing Site is the place where your rocket and the three Onions land at the beginning of each day. There are always strange 'landing pad' patterns on the ground beneath each onion. Pikmin that are left in this area at the end of the day will return to their onions automatically unless they are drinking Nectar.
In the Wistful Wild in Pikmin 2, there is what appears to be a landing site between the Dream Den and Hole of Heroes. It is actually what used to be the landing site at The Impact Site in Pikmin. The markings and surrounding area are the same. See more Wistful Wild and Impact Site comparison pictures on their pages.