These are neat. They are from WWI, circa about 1917. The US Food Administration, which eventually became the FDA, was involved in rationing food so that there was more food available for troops, as well as keeping enough for the rest of the country to eat.
So these signs urged special days of the week to go without certain food items. "Meatless Days, Wheatless Days, and Porkless Days". These signs went up at restaurants and stores to tell patrons that they participated in the 'rules' set up by the Food Administration, though I think they were more suggestions than actual requirements.
Might not be a bad thing today to have meatless days on occasion.