I thought that some one earlier had mentioned the fact that "Norma" was chosen as Charlie's sister's name. I couldn't find the post, but have to confess that it was not an original thought, and didn't occur to me during the reading.
The change of season is something that I dearly love in my area of the country, and is something I look for and identify with in reading. Keyes has Charlie's surgery performed in early March, just before the emergence of spring. Through the spring and summer, his intellect flourished and he grows mentally and emotionally. By the end of summer, Charlie (per Erikson's developmental stages) has achieved "maturity" - understanding that "love and work" are necessary. As the days shorten and grow colder and more dreary, Charlie checks himself into Warren to spend his remaining days in the cold isolation of his own mind's "winter."