Given that you never know what the weather is going to do at that time of the year I would have thought that you should choose September with confirmation say 48 hrs before departure and use October as a fall-back possibility if Sept doesn't work.
I also agree that Cherbourg is a safer destination if large numbers are involved and if anybody wanted to make a long week-end of it you could do a triangular couse: Cherbourg then St Vaast.