I can say that many of them are rare and when they happen, I can't keep track of the time when the MWC is active, in my case I'm looking for the bank stolen for months on my server and I can't find it active.
In tibia Fandom talks about some Statistics: "Through some empirical research it became very likely not every Mini World Change has the same chance of occurring. Therefore statistics have been gathered which can be seen below. This is a working project since more statistics need to be added to get accurate data. To gather this data a character needs to check The World Board once every 24 hours, which can be done on multiple worlds. A total varying between zero and thirteen active MWCs at the same time have been encountered."
Mini World Changes | TibiaWiki | Fandom
I don't know if there are any sites that we can observe the day a mini word change was activated and its probabilities. I believe that events are random.