Posada in Spanish can mean inn, lodging, shelter, boardinghouse, home, etc., depending on context. Traditionally in old Mexico, when people were traveling, at the end of the day they would seek a place to spend the night out of the elements. When no inns or hotels were available, travelers would seek “posada” in private homes, asking for their hospitality and kindness and sometimes receiving a meal with the resident family. Often they simply slept on the floor. As the population became converted to the Roman Catholic faith by the Spanish Conquistadors, a tradition developed based upon the Biblical story of Joseph and Mary arriving in Bethlehem on the eve of Jesus Christ’s birth.
In neighborhoods (barrios, colonias) across Mexico, beginning on the 16th of December you will find groups of friends participating in the tradition of “La Posada”. Each night for nine nights, a different home is selected as the “Inn” for that evening, and the other participating members of this group gather at another place and proceed through the streets to the selected “Inn”. Sometimes they are carrying images of Joseph and Mary, and other times they have two of their members dressed in costumes representing them. The hosts have prepared their home for that night’s gathering, usually making tamales, ponche, atole and other food and refreshments for the participants, a piñata with candy for the children, and other festive decorations.
The “travelers,” representing Joseph and Mary seeking a place to spend the night, begin to sing outside the chosen host home. The traditional song has twelve verses that are sung back and forth in a question/response format between the people outside and the people inside, plus a final verse they all sing together as the travelers are finally invited into the host home. The English translation is available online here: http://gomexico.about.com/od/christmas/a/posada-song_2.htm and the Spanish lyrics are also available through a link on that page.
It is said that the nine nights of Posada represent the nine months that Mary carried Jesus in her womb, and also represents the nine days she and Joseph traveled to get to Bethlehem. The final night of Las Posadas is Christmas Eve, referred to in Mexico as Nochebuena.