Colleges all have different dates. There are some that keep taking the common app well into spring and even summer.
Some high school staff members kick into self preservation mode and set arbitrary dates that applications need to be submitted by just to survive the holidays. After this first round of ED, EA, and rolling admissions notifications, there can be a flurry of new applications for RD and ED2 that require additional steps for staff just as the holiday break begins. I can see a school counselor setting that as a date to have everything submitted by.