That reminds me of something I found in Pokémon Sword/Shield four years ago: on the 29th of February, the entire Wild Area will have foggy weather. The weather pattern is different for each day of the year (repeats each year), and is never that uniform for any other day AFAIK.
Nice little Easter Egg from the developers. “Yeah, we remembered that February 29th exists and did something about it. Now go catch a Milotic or something”.