Well, it's two sleeps, regardless. If two nights have passed, then the notice requirement will have been fulfilled.
So if I send it, say, at 7 o'clock on a Monday evening and we have a meeting on a Wednesday, then that's fine because you have the Monday night, you have the Tuesday night, and the meeting is on the Wednesday. So two sleeps is the rule, in the absence of an instruction from the committee. Obviously it's up to the committee to decide its notice requirement.