The middle of the 20th century was considered to be the "golden age of food processing", as what was coined by Harvey Levenstein, because the American believed at that time that "processed foods made them life easier." To add up, these methods of processing meat and freezing was one of the scientific innovations achieved at that time.
You can conclude that Rita enjoys being active and being around friends and family, so when she didn't have anything to do the rest of the summer she grew bored and lazy, which caused her to count and dwell on the fact that her friends weren't with her and how many days were left until they were.
