Even if they're inclined to spend the day watching college football, birthdays and the first day of school are more significant events because children's lives are governed primarily by an academic calendar.
This changes in adulthood, because our lives governed by tax and fiscal years that tend to align with the calendar. This incongruity is helpful to a parent, as it let's us consider what age & grade they'll be, and what skills & capabilities they'll have, when the year is out.