Joseph Smith recorded that Oliver Cowdery acted as his scribe from April 7, 1829 onward.
Joseph Smith-History, Pearl of Great Price, The Church of Jesus Christ of Latter-Day Saints, accessed January 19, 2022
66 On the 5th day of April, 1829, Oliver Cowdery came to my house, until which time I had never seen him. He stated to me that having been teaching school in the neighborhood where my father resided, and my father being one of those who sent to the school, he went to board for a season at his house, and while there the family related to him the circumstances of my having received the plates, and accordingly he had come to make inquiries of me.
67 Two days after the arrival of Mr. Cowdery (being the 7th of April) I commenced to translate the Book of Mormon, and he began to write for me.