DS1's junior school always send letters about class assembly, meet the head coffee mornings etc exactly 1 week before the event.
I really do try to support everything the school does and am in the fortunate position that I can arrange my work diary to suit me, but once something is booked in, moving it means letting down a customer.
I've had a letter today that DS2's "moving up" session from Infants to Juniors is this time next week. I obviously don't want to miss it, but really a week?
Someone at work said schools do this deliberately because they don't want too many people turning up, but that may be over cynical....