Summer rain is a rare weather event in Davis. Between June and September the area is dominated by subtropical high pressure, which shunts precipitation-producing weather systems well to the north, so rainfall during those months is negligible, if not absent.
It occurred on September 20 and 21 2013. (barely summer, it should be noted — Autumn began on September 22 in this year, but Summer anyway). That day, September 21, around 6.15 PM there was a complete cloudburst including hail & thunder. The hail was observed for about 15 minutes.
It occurred on August 5, 2014 too and in the night of 5-6 August 2014. Because this rain was very mild and because of the exceptional drought California is in, unfortunately, it will not bring any substantial relief.