The household family calendar–digital.
Every year as a family we bought buy a paper "family" calendar. One of those ones that has 5 columns in our case. Enough columns for each member of our family, plus a spare.
A monthly calendar that allows us to mark events, activities, holidays and more to a family member, or the "family" column (remember that spare column).
This has worked wonders for a number of years–until it doesn't.