The Hakurei Shrine Maiden had this brilliant idea to host a Christmas themed festival in the human village, and it immediately caught on! All of the residents of Gensokyo, human or youkai are invited to celebrate Christmas and have fun in any way they can! You're out on a date with Sakuya and you're there to have fun with her as you explore the various sights and attractions going around in the festival!

[From Steam]