In my case that is May 1, 2015. The only way to solve this would be storing the date on which a person made their first upload. Let me just add a short comment of my own: I really had no idea in which month I took the most photos. That is why, for me, the clear winner is March, when it is usually still winter, surprisingly. It has almost 2x as many as the months in 2nd place. For May you need to subtract 865 from the first table (the first upload of photos without EXIF data), which leaves you with 409 photos, rather few for summer. September and November are certainly interesting, since it is warmer then than in winter or spring, yet there are several times fewer photos.
I was also surprised by the higher number of photos throughout the whole winter.

Only 2 hours have passed since I wrote this post, and I have finished the last table and wrapped up a few small things. So if anyone would like to take a look at my statistics, they are at https://photos.sunamo.cz/en/en/statistic/1.