"It was on the morning of a beautiful, clear day,
early in the spring of eighteen hundred and twenty" (JS-H 1:14).
Joseph Smith's "first vision," the foundational event of the restoration, may have been in April 1820. We do not have any contemporary documentation regarding this event, or any subsequent documentation from Joseph regarding the date of this event. We do, however, have Orson Pratt stating almost fifty years later that it occurred in April of 1820:1
Since Joseph Smith was born December 23, 1805, Elder Pratt's estimate of "fourteen years and four months old" places Joseph in approximately April 1820. Almost a decade before Elder Pratt's comments, Jules Remy and Julius Brenchley, two visitors from Europe, visited Salt Lake City and wrote of their experience with the Latter-day Saints. In 1861, they published A Journey to Great Salt Lake City, in two volumes. Interestingly, included as the "Chronological Summary of the History of the Mormons," was the following:2
Whether or not Elder Pratt's memory is correct, and while the source of Remy and Brenchley's chronology is uncertain, it is interesting that two sources date the first vision to approximately April 1820. One wonders whether the first vision might have actually occured on April 6, 1820.
1 Orson Pratt, "Mormonism" (Feb 24, 1869), Journal of Discourses, 12:353 (available online here).
2 Jules Remy and Julius Brenchley, A Journey to Great Salt Lake City, 2 Vols. (London: W. Jeffs, 1861), 1:503