It's a great thing that we have been keeping records in California for a long time.
We can measure and record all sorts of things these days, but the main things we have been keeping track of in some locations since the 1800s is the high and low temperature and precipitation. This way when something seems unusual, we can look it up and see.
June rain surely seems like an odd thing. The days are often warm and sunny with the most amount of daylight in any month. Rain hardly seems like something we remember as school is getting out and we are thinking about beach vacations or hiking in the mountains.
We do know, though, that rain in valley locations in July is certainly a rare thing with an average of 0.00" in the Sacramento area, but June rain is often recorded.
Above is the climate for the Sacramento with an average of 0.21" for the month of June going back at least 30 years.
Stockton is much less with 0.08" for the month of June. This is typically as rain totals in California usually go down the further South you go, and up the higher the elevation.
So to summarize, June rain seems like it's rare but you typically get one weak June storm and this may be it.