"Maybe you can go to Italy this summer."
Translation:Belki bu yaz İtalya'ya gidebilirsin.
Well, it doesn't apply to "yaz" and "kış." Many (but not all) time expressions stay in the nominative.
The seasons "summer" and "winter" also use the more archaic forms "kışın" and "yazın." These just have to be memorized as exceptions. If you didn't use "bu" in this sentence, you would have used those.