(CNN) Students often joke about the "mystery meat" in their school lunches. Well for kids in New York City, there won't be any mystery at all, at least on Mondays.

That's because all public schools in New York will have "Meatless Mondays" in the 2019-2020 school year. New York Mayor Bill de Blasio introduced the new meal program Monday. Meatless Mondays, which will provide students with all-vegetarian breakfast and lunch offerings, is being expanded citywide from a pilot program that was tried out last spring in 15 schools.

"Cutting back on meat a little will improve New Yorkers' health and reduce greenhouse gas emissions," de Blasio said at a news conference. "We're expanding Meatless Mondays to all public schools to keep our lunch and planet green for generations to come."

De Blasio also said parents could still send their children to school with a lunch that contains meat if they liked.

Proponents of Meatless Mondays point to the health benefits of eating less meat.

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