Young left the Basie band in late 1940. He is rumored to have refused to play with the band on Friday, December 13 of that year for superstitious reasons spurring his dismissal, although Young and drummer Jo Jones would later state that his departure had been in the works for months. He subsequently led a number of small groups that often included his brother, drummer Lee Young, for the next couple of years; live and broadcast recordings from this period exist.  During this period Young accompanied the singer Billie Holiday in a couple of studio sessions (during 1937 - 1941 period) and also made a small set of recordings with Nat "King" Cole (their first of several collaborations) in June 1942. His studio recordings are relatively sparse during the 1942 to 1943 period, largely due to the recording ban by the American Federation of Musicians. Small record labels not bound by union contracts continued to record and he recorded some sessions for Harry Lim's Keynote label in 1943.  In December 1943 Young returned to the Basie fold for a 10-month stint, cut short by his being drafted into the army during World War II (see below). Recordings made during this and subsequent periods suggest Young was beginning to make much greater use of a plastic reed, which tended to give his playing a somewhat heavier, breathier tone (although still quite smooth compared to that of many other players). While he never abandoned the cane reed, he used the plastic reed a significant share of the time from 1943 until the end of his life. Another cause for the thickening of his tone around this time was a change in saxophone mouthpiece from a metal Otto Link to an ebonite Brilhart. In August 1944 Young appeared alongside drummer Jo Jones, trumpeter Harry "Sweets" Edison, and fellow tenor saxophonist Illinois Jacquet in Gjon Mili's short film Jammin' the Blues.

Answer this question "Why did he leave?" by extracting the answer from the text above.
He is rumored to have refused to play with the band on Friday, December 13 of that year for superstitious reasons spurring his dismissal,