From the Rain Day website:
Rain Day got its beginning in the Daly & Spraggs Drug Store, located in the center of Waynesburg. Legend has it that one day a farmer was in the drugstore and mentioned to Byron Daly that it would rain the next day, July 29. Mr. Daly asked him how he knew and he replied that it was his birthday and that it always rained on his birthday. He had a journal for several years in which he recorded the weather and always had noted rain on July 29th. Mr. Daly thought this was too sure a thing to let pass, so he started betting salesmen who came into his drugstore that it would rain in Waynesburg on July 29. The bet was usually a new hat, which of course he would win.It's apparently rained on 113 of the 137 "Rain Days" in Waynesburg so far.
With so much of the United States in drought right now, The Blog of Days wishes that this was one day that could literally catch on nationwide.