I half-agree, as it's not only about not eating meat that day. It also makes people realize that there's actually good dishes you can do that taste good without a big, fat piece of meat. That might be obvious to you, but many people still don't accept that fact. Having a meatless day offers an opportunity to showcase vegetarian lifestyle to non-vegetarians. And once people realize that "hey, that's not as bad as I've thought!", they're gonna make vegetable stir-fry next Saturday too.