To stay on top of everything, I maintain the universal family/work calendar on Outlook in my office.
In that calendar, I pop all of dh/my dcs' school, work and social events such as:
Term dates
School hols, my/dh hols
Birthday parties
Which days I/dh am late/not coming home or travelling
Days when the aupair is not around
mufti/costume days
Playdates
School plays, sports days
Afterschool events, extra curricular activities
School project due dates
Dates of tests
Things I have to buy - esp those bits of school uniform that routinely get lost/outgrown
For each event, I set a reminder e.g. birthday parties I set a week reminder so I have time to buy and wrap a present and get dcs to do a card. If I have to get a costume, I put a 4 week reminder to give myself time to find one. Mufti days I put a day's reminder so I know to remind dcs/aupair for the next day. Or for school plays, I put reminders to myself/dh to book leave.
For those days that affect the aupair, I also put it in the family calendar at home, so even if I forget to tell the aupair, she can read it.
The diary is the command centre. Dh does not book anything over the weekends without first consulting me re: the calendar ...
I would say that generally I am on top of things and the household runs like clockwork