So - I'll see you at the restaurant on Saturday. Oh, by the by, is it BYOG.?
BYOG : bring your own grog (=alcohol)
Most restaurants these days have a liquor licence. In 'the old days', if a restaurant didn't, people would bring their own bottle of wine and the restaurant would charge a nominal fee for 'corkage' - opening and serving the wine in glasses they provided. Unless one had been to the restaurant, one had to check - otherwise, no grog.
En passant, laterally connected with the main subject. “By-play” is side or secondary play; “By-lanes and streets” are those which branch out of the main thoroughfare. The first “by” means passing from one to another, as in the phrase “Day by day.” Thus “By-the-by” is passing from the main subject to a by or secondary one.