It was a regular game, day 18, cleaning up. Suddenly, this misunderstanding comes flying around the corner and, naturally, I ran away. I could save the game when I looked at it, which is quite strange. No less strange is that when I was saved (the save icon below is where I am on the roof of the garage, and the ball is in the door) I decided to touch this thing... it turned the mirror mode and i was thrown out to the river, where there is a wooden village behind the fence.

The ball was yellow, sparkling, and there were special lightning effects. I searched the wiki for what it is, but I couldn't find it. What was that all about?

You ran into a Wisp, there's various Wisps, Orange teleports you somewhere on the map and enables Mirror Mode. There are some Wisps that are actually dangerous though, so....just a warning.

https://voicesofthevoid.wiki.gg/wiki/Entities/Wisps

For some reason, it seemed to me that all Wisps, without exception, appear only from 26 to 31 days...

they are rare but they can spawn. theres (atleast) one fixed event that dumps a crap ton of the bad ones though.