Does it sound better if I say:
... hope you have marked your calendars
instead of :
... hope you have blocked the date
for a certain event in the future for which they have already been notified ?
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I definitely wouldn't ask anyone to block their calendars. Blocking should be more associated with pipes and toilets than about people's arrangements, in my view.
Besides I think it sounds officious to ask people to block something - it sort of assumes that you are obligating them to participate in whatever it is that is happening. Asking them to note something makes no such imperious assumption.
In Britain we would normally ask people to note it in their diary or make a note in their diary.
I don't think Americans keep diaries but calendars. To us, a calendar is something you hang on the wall. A diary can either be in old-tech notebook form, or something you keep on your mobile phone.
"I hope your calendars are blocked" is an alternative depending on the audience you are referring to.
Assuming - event is formal audience is higher management
"I hope you have marked your calendar for event. If not, please block your calendar as your presence is important"
Assuming - event is informal, team activity, et al. audience is peers "Please block your calendar for this event. Ignore this reminder, if already done"
Hope this helps.
NOTE: in the above suggestions, 'blocked' can be replaced with 'marked' :)