I’m not going to pretend to be an expert on names, but I do speak a little German, and the internet is full of interesting information. Most of you probably know that “Sommer” is German for “Summer”. So far, so good, but why would you call someone that?
This Website points out that adding an extra “er” on to a German word signified one of two things. Either it was appended to a place name (a city, town, castle or geographical feature) to indicate that you were from that place or it was appended to indicate an occupation:
Surnames which are derived from a trade, profession, or occupation, or some product or tool distinctly associated with a particular craft, for example: Amtsmann (official), Bauer (farmer/peasant), Beck/Becker (bake/baker), Deck/Decker (roof/roofer), Ferderer/Föderer (plaintiff), Fitterer (feeder/provides fodder), Germann (spear + man), Hellmann (warrior), Kessel (kettle), Koch (cook), Nagel (nail/peg), Schröder (tailor), Zander (Alexander/tooth extractor), Zentner (tithe/tax collector), Zimmermann (carpenter/finish carpenter), etc.
It goes on to point out that the double “erer” ending could indicate someone who was apprenticed to someone from a certain place or trade.
This Website actually jumps right to the meaning of Sommerer:
German (Austria): variant of Sommer, a nickname for a summer tenant or a worker hired for the summer.
But I don’t know how reliable that is. It is the internet.