September 29, 1952: The New York Stock Exchange ended the Saturday trading session to create a five day work-week. To even out the reduced number of days, a half hour was added to the close of the Monday-Friday trading sessions. That made the sessions open from 10AM -3:30PM.
Sources: MarketWatch, Friends of Business History
Read more about NYSE in our collection:
Chairman of the Fed : William McChesney Martin Jr., and the Creation of the Modern American Financial System
A Financial History of the United States
Fundamentals of Financial Instruments: An Introduction to Stocks, Bonds, Foreign Exchange, and Derivatives