My dermatologist only makes appointments 3 to 6 months in advance. Same with my pulmonologist. I heard psychologist appointments are often enough between one and two years in advance (if you want to start a new therapy).
Vacation is another such thing. Some employers require you to plan your vacation time at the beginning of the year so they don’t end up with a whole department being gone at the same time. Of course I need to factor that into my calender then so I don’t put other appointments/promises in there.
Weddings and large birthday parties are also often announced and planned far ahead. I need to block that in my calender as well.