Honestly, this idea that local events "unify" anything between travel planning and weather is mainly a good sales argument for those who have an interest in getting you to go to a destination at a specific time.
The motive behind these claims is often commercial, encouraging bookings for specific dates.
For example, during the Lille flea market, I didn't see the weather aligning with my parking issues or hotel prices skyrocketing; I mostly saw vendors making good deals.
It's more about maximizing profits for local actors than a real harmonization for the average traveler.