Its a touhou reference
In the underworld there are the phantoms of dead people in a place called the sanzu river, eiki is who decides if you go to heaven or hell, eikis servant makes sure that the phantoms dont try to escape from eiki's judgment. But her lazy servant komachi, sometimes forgets and lets them free into the over world. The phantoms take the role of a flower and no matter the time of year it would cause random flowers to start appearing. This happens every 60 years depending on the phantoms and if komachi is doing her job. and the people of gensokyo call this the "flower veiwing incident"
Funny paragraph of random lore
