They call it Fall Fest on some worlds, and Autumn's Progress on others. Everybody flocks to the northern hemisphere on our planet and celebrates a new season. Even though most of them come from tidally-locked worlds where they're on continents that're always in deep summer. I guess people just want an excuse to party.
Sergey Zabelin is an art director in Ukraine who works on videogames. See more of his work on his CGHub gallery.
For the complete effect on that image above, you can see it full-size here.