I'm with you to a point op - there are plenty of dates that can be put on the calendar in advance - obviously term dates (and it's nice to have next year's term dates too), open evenings, the Christmas concert, the school play, sports day (plus a couple of spare dates if it's wet), the summer fete and the Leaver's Service for example - most of these will have set dates anyway even if just in terms of the Christmas carol concert is on the Thursday and Friday of the last full week of the winter term.
If you put the main dates in (and full details - dc's school is getting better at putting things in the online calendar before they happen but they just put a title, times if we are lucky, but no useful details!) then it's fine to add other things in later, especially with a couple of months to spare.
It would also be useful to have a list of things that you expect to happen and expected dates and costs - so things like all the trips (y7 - history trip to Warwick Castle in November, last year's cost £20, y8 trip to British Museum in March, expected cost approx £15-25, mock exams in January, we usually support Red Nose Day (month xx) and children in need (month yy) and give pupils a vote on a third charity day to support. All details subject to confirmation which will be put on the school calendar and notified through the school news page on the website and in the newsletter. Any updates, changes, additions or cancellations will be notified in the same way.
When I was a school we had a 'blue book' which had everybody's name, house and class in, all the teachers, all the term dates and most of the other dates for events that happened through the year - there was also space to add a couple of extra things that might arise during the course of the year. Plus there were blank timetable pages to fill in, useful reference info and some school facts. If it was possible to create something like that in the pre internet age then it should be easier now!
It's true that the parents aren't customers but if parents aren't contacting school when they have a problem but grumbling on social media then that's a massive red flag that communications between school and parents are failing, regardless of how good the school is seen to be.