After I wrote about a failed project to use Google to find the best month to visit somewhere, I went back, had a look at the code and tortured it until it gave me reasonable answers. It is here: best time to visit
The code works by searching for the names of months in the returned google descriptions. The two top months are assumed to be the month between which you should visit. One problem is determining which one comes first. For Amsterdam and Australia, the code returns April and October. But you shouldn't go to Amsterdam in februari, if june is an option.