Learn about it at the New York Public Library's new exhibit, Lunch Hour NYC:
[I]t is a former snack that only became the day’s third fixed meal as society urbanised and industrialised, and workers were unable to return home for dinner (always the main meal of the day) until late at night. As [Laura Shapiro, culinary historian] explained:
Lunch came into its own — it really acquired the size and shape and substance that it has in America today — in New York. New York is emblematic, arguably, of large North American manufacturing cities — it has all the conditions that make America different from the Old World in terms of speed and work and the arrangement of life. In New York, the focus of people’s lives is work, and lunch is the meal that was just made to fit into the industrial, urban work day.
The Dish 