Rain doesn't usually have much effect. In a few rare cases rain can change the PH a bit, but that is not common. Extreme amounts of rain that cause the pool to overflow will lower some levels, most importantly salt and CYA, but again that is fairly uncommon.
You should backwash each time the filter pressure goes up %25 from the clean filter pressure.
There are no such thing as dumb questions. We all needed to learn these things at some point.