Okay just a clarification, I see that Tomo's birthmonth is in september, and the reason for it is because of the summer thing. I just wanna clarify that summer in Japan starts somewhere between June 22-23 and the vacation starts in July. In the book, when they celebrated Tomo's birthday, they were still in school so there is no possible way that her birthmonth would be in September. Maybe somewhere near Jun22-Jul21. Plus vacations didn't really immeadiately start when they celebrated Tomo's birthday.
I'm sorry if there are any grammar mistakes. Please excuse me for my first language is not english.