Isn't it a bit quick to generalize the impact of local events on travel planning? In our context here in Portugal, the relevance of an event depends heavily on the specific destination and the traveler's intent.
For example, for a pilgrim going to Fátima, the date of a religious event is the reason for the trip itself, it's decisive.
However, a tourist coming to relax on the beaches of the Algarve won't care much about a music festival in Lisbon; the impact will be null for them.
Most often, practical constraints or the local climate are much more decisive for the vast majority of travelers.