Short answers, pulled from the story.
Union General Gordon Granger arrived in Galveston Texas on the 19th of June 1865. He brought a handwritten document signed by Major F. W. Emery that declared all slaves free within the state of Texas.
The National Archives holds the only surviving handwritten record of General Order No. 3 inside Record Group 393: Records of U.S. Army Continental Commands. The document resides physically within the National Archives Building in Washington D.C.
The delay occurred because no Union military force existed in the region to enforce the law before General Gordon Granger's troops landed at Galveston. Without soldiers present slaveholders ignored the proclamation without consequence until federal power arrived.
The Galveston Tri-Weekly News published the text of General Order No. 3 on the 20th of June 1865. That publication appeared just one day after General Gordon Granger posted the notice in town.
The Thirteenth Amendment ratified and proclaimed in December 1865 made slavery illegal nationwide across all fifty states. General Order No. 3 simply enforced the earlier proclamation within Texas specifically while the amendment provided universal abolition.