Oliver Cowdery says he first met Joseph Smith on April 5, 1829; describes Book of Mormon translation process.
Oliver Cowdery, Letter to W. W. Phelps, in Latter-day Saints' Messenger and Advocate 1, no. 1, (October 1, 1834): 14
Near the time of the setting of the Sun, Sabbath evening, April 5th, 1829, my natural eyes, for the first time beheld this brother [Joseph Smith]. He then resided in Harmony, Susquehanna county Penn. On Monday the 6th, I assisted him in arranging some business of a temporal nature, and on Tuesday the 7th, commenced to write the book of Mormon. These were days never to be forgotten—to sit under the sound of a voice dictated by the inspiration of heaven, awakened the utmost gratitude of this bosom! Day after day I continued, uninterrupted, to write from his mouth, as he translated, with the Urim and Thummim, or, as the Nephites whould have said, “Interpreters,” the history, or record, called "The book of Mormon."