"Mary Don't You Weep" has had a long history, dipping in and out of popularity throughout several periods. It's fitting then that several different settings could be identified for the song, including Old Testament Egypt, New Testament Galilee, 1870s Nashville, and the 1950s American South. The song was composed in slave communities during the decades preceding the Civil War, though, so this may actually be the song's most appropriate setting.
The portrait of America's antebellum slave communities has changed over time. Throughout the 19th century, many historians embraced the picture of slave life painted by Southerners. Slaves were supposedly content in their lives and well cared for by their masters. According to those who felt the need for a moral justification for slavery, slaves were unambitious and unable to fend for themselves, so slavery actually provided a necessary paternal environment.
Riiiiiiiight.
Over the course of the 20th century, this benign portrait of slavery was challenged. (There were certainly challenges to this presentation before, during, and after the Civil War as well, but many Americans in the North had never been to a Southern plantation and had no firsthand experience with the conditions slaves were living under.)
Shortly after World War II, many historians began to embrace a very different portrait of slave life. They began to realize that slavery had been overwhelmingly brutal, often stripping slaves of their physical and psychological defenses. Many found the brutality of plantation life too much to endure. Slaves were "infantilized," turned into dependent children unable to protect themselves or function like adults.
During the 1970s, this understanding of slave life was challenged by a number of historians who argued that slave communities were healthier and less devastating than scholars suggested. These historians argued that slaves were not emotional cripples and that they found numerous ways to resist the brutalizing conditions of slavery. They formed healthy relationships, identified their own leaders, and established customs that bound them together in semi-independent communities.
Within these communities, stories and songs provided a source of cohesion, even rebellion. These stories and songs were often coded so that their meanings would be hidden from slave-owners but immediately understandable within slave communities. One told fugitive slaves how to evade bloodhounds ("Wade though the Water"), one was used to announce the arrival of a conductor on the Underground Railroad ("Swing Low, Sweet Chariot"), and another provided Mississippi slaves with a detailed map north to Illinois and freedom ("Follow the Drinking Gourd"). Stories about Brer Rabbit provided endless accounts of a wily animal breaking rules and tricking more powerful figures.
The tales implicitly ridiculed slave-owners and inverted the location of real power in the community. Other spirituals played a somewhat different role. Many, like "Mary Don't You Weep," offered solace and promised that eventually the world's injustices would be corrected and that the slaves' oppressors would be punished.