http://offpeak.io the project my team has been working on aims to solve this exact problem. We help travelers avoid peak season, and peak dates that are often associated with large business conferences.
It seems to be missing all but the largest cities. My guess is that you are manually adding data which would mean higher quality at the cost of general usefulness.
I used an Instagram heatmap to find places to visit/look at during a holiday, I would guess if one adds a "time heatmap" one can see which weeks/months are quieter and which are busier.
The search is really frustrating. Everything I type is not there and then it shows me things that are very far away from what I typed. It may be better to show a list organized by region or show the closest location you do cover to the one that was typed in.
