January 6, 1842 - Joseph Smith (Age: 36) married Agnes Coolbrith (Age: 33), who was the widow of Don Carlos (Joseph's brother).
"Agnes Coolbrith was baptized in Boston in July of 1832 and gathered with the saints in Kirtland, Ohio. There she boarded in the home of Joseph and Emma Smith. In 1835, Agnes married Joseph’s brother, Don Carlos. Three years later, when Joseph fled Kirtland for Missouri, Agnes and Don Carlos followed. Eventually they settled in Nauvoo with their three daughters. During the summer of 1841, Don Carlos contracted malaria and passed away." (In Sacred Loneliness: The Plural Wives of Joseph Smith pgs 146–153)
"Five months later Agnes would marry Joseph Smith. The marriage was guarded with secrecy. On January 6, 1842 Brigham Young wrote a cryptic entry in his journal using Masonic symbols. Decoded, it reads: “I was taken in to the lodge J Smith was Agness”. The abbreviation “was” means “wedded and sealed”. On the same day in Joseph’s diary we find: “Truly this is a day long to be remembered by the saints of the Last Days; a day in which the God of heaven has began to restore the ancient order of his Kingdom...all things are concurring together to bring about the completion of the fullness of the gospel.”" (In Sacred Loneliness: The Plural Wives of Joseph Smith pgs 153,154)
"Later that spring, at a meeting of the women’s “Relief Society”, Emma Smith announced that a young woman, Clarissa Marvel, “was accused of [telling] scandalous falsehoods on the character of Prest. Joseph Smith”. Eager to prove her husband innocent of improper behavior, Emma initiated an investigation. A few days later, Clarissa signed the following statement: “This is to certify that I never have at any time or place, seen or heard anything improper or unvirtuous in the conduct or conversation of either President Smith or Mrs. Agnes Smith...I never have reported any thing derogatory to the characters of either of them”. By this time, Joseph and Agnes had been married almost four months." (In Sacred Loneliness: The Plural Wives of Joseph Smith pgs 154,155; Mormon Polygamy: A History pgs 20,21)
"After Joseph Smith’s death, Agnes married Joseph and Don Carlos’ cousin, George Albert Smith. As George Albert and the rest of the Saints were leaving for Utah, Agnes wrote, “I have no other one to ask but you my mind is much troubled about comeing...I want to come and I do not want to come I feel alone all alone if there was a Carlos or a Joseph or Hyrum there how quickly would I be there”. Agnes ultimately did not migrate to Utah. She married a fourth husband, William Pickett, and eventually moved to California, essentially leaving her Mormon past behind." (In Sacred Loneliness: The Plural Wives of Joseph Smith pgs 157–159)
"Many years later, her marriage to Joseph Smith still shrouded in secrecy, Agnes wrote to her nephew, Joseph F. Smith: “I acknowledge none greater...than those that belong to the household of Joseph our Dear Dear Dear departed one Joseph...I could say many things to you...that I know and that has been told me by those that are dead and gone but perhaps you would not believe me no I know that you would not so it is best for me to keep silent.”" (In Sacred Loneliness: The Plural Wives of Joseph Smith pgs 166–167)
Source Material: http://wivesofjosephsmith.org/07-AgnesCoolbrith.htm