Before the modern MLB trade window, the uniform trade-deadline date adopted before the 1923 season was June 15.
That June 15 cutoff governed MLB trades from 1923 through 1985. It was intended to limit late-season roster stacking and preserve a more stable competitive balance once the season was underway. Before this rule, clubs had used several different restrictions, including waiver requirements, rather than one universally recognized midsummer deadline.
The date is often mixed up with August 1, an earlier National League waiver-related date, and with July 31, the deadline that replaced June 15 in 1986. June 15 is the historically correct answer for the long period beginning in 1923, not the date associated with the modern trade calendar.
MLB’s deadline structure continued evolving. The 1986 change extended the trading period deeper into the summer, and later reforms eliminated the separate August waiver-trade period. Those changes explain why different dates appear in historical accounts of baseball transactions.