Everything is considered a brand these days. Why not summer? It has the characteristics of a brand: Brand attributes – heat, long days, short pants, time off. It has a brand's personality – informal, gregarious, fun-loving, nostalgic. It has its own sounds (katydids in the afternoon), sights (fireflies at dusk) and smells (sea salt or cattle feed lots or steaming bags of trash, depending on where you live).
The only reason that summer would not be considered a brand is this: No one is actively measuring and managing its brand value... There are no brand consultants for summer... No publicists... No logo designers... Summer just happens, and no one is in charge of it... but you.