If by "release date" you mean when a record was available in retail stores, I am guessing it varied across the country. Pacific Jazz releases were available earliest in Los Angeles where the local distributor, California Record Distributors, received stock from the pressing plant and filled orders from local stores. CRD would then ship to 30+ other distributors across the country. Down Beat was probably the best source regarding availability as Bock would place ads to promote new releases anticipating that records would be in the stores when the ad appeared or shortly thereafter. The Gerry Mulligan Songbook was recorded in New York in December of 1957. Woody Woodward dated the test pressing when he received it, January 29, 1958. The LP was included in a fullpage display ad in Down Beat on April 17, 1958. The Mulligan Songbook was the first stereo release in the new ST-1000 series. Woody dated the test pressing on June 30, 1958. The stereo version was featured in a display ad in Down Beat on September 2, 1958.