I learn new vocabulary by trying to relate to its component parts, or something that sounds like it in English, or when all else fails a silly mnemonic. When I see the word for Summer I parse it as good (καλό) weather (καιρώ).
I didn't know καιρός could mean "weather." I normally associate it with time, especially opportune time, as it is used in koine Gk (Mark 1:15). But even in ancient Gk it could refer to a season, so the modern Gk sense of weather makes sense. When I first came across this modern Gk word καλοκαίρι I thought it was great that the word for summer is "good time." I guess it refers to the opportune time/weather to plant crops.