Context first: the impact of local events on travel planning is not a universal truth everywhere.
Here, in Portugal, it depends heavily on the type of traveler and the destination.
For a tourist seeking big cities, a festival can drastically affect the budget and experience, like during Santo António in Lisbon.
But for those prioritizing calm or small regions, like a stay in a remote village in Alentejo, a local event will have only a marginal impact on their choice, or none at all.