...the first of September. Sir William was a Shot & could not support the idea of losing such a Day, even for such a Cause.

I assumed that September 1st had something to do with hunting, what with the reference to being a good shot. Some google searching shows that September 1st is the Opening Day of the game bird hunting season. I suppose that in Sir William's case the hunting season is a more rare occurance than his falling in love, so he can't pass up the opportunity - LOL!