Corrected entry: Throughout the movie there is an Advent Calendar that is opened from time to time, revealing what day of December it is. However, in the scene where the grandmothers are sitting in the den and one of them is decorating a gingerbread house, they are watching the Macy's Thanksgiving Day parade, which takes place in November.
Corrected entry: When Cousin Eddie barges into the house with Clark's boss, Ellen jokingly tells him this is the family's first kidnapping, which is actually wrong. In National Lampoon's Vacation (1983), Clark holds the security guard at Walley World hostage, and forces him to take the family on the rides.
Corrected entry: We later learn that Eddie's RV makes a lot of noise when driven. How did they ever make it into the Griswald driveway without anyone noticing when they first arrive?