McDonald's will sell the tsukimi burger again next month. For a long time after coming to Japan, I kept seeing the tsukimi burger offered over and over again. Deja vu over and over as they say. It certainly has become a tradition, this article says that McDonald's Japan first started selling it in 1991 as a seasonal offering.
As can be seen the the picture, the tsukimi burger employs an egg in the same form seen on Egg McMuffins. Tsukimi means viewing the moon in Japanese, so I suppose the yolk is supposed to represent the moon, although the round shape of the egg might also be what is being referred to.