The amendment itself was adopted back in 1865 in the United States. 3/4 of the states were needed to ratify the amendment, so the U.S. did not need Mississippi's approval to add the 13th Amendment to the U.S. Constitution. In 1994, a clerk in the Texas legislature had discovered that Mississippi hadn't ratified it yet, so an effort began in Mississippi to do that. He mailed letters to all the African-American members of the Mississippi legislature, letting them know this. On March 16, 1995, Mississippi finally ratified the 130 year old amendment. (Nashua Telegraph)
Of course, this was only important symbolically. You still weren't allowed to own slaves in Mississippi from 1865 until 1995. Learn more about the 1995 effort to ratify the amendment here. Also check out this list of states with the dates of when they ratified the amendment. Kentucky didn't ratify it until 1976. Delaware didn't until 1901.