If you've ever spent time in Mexico during the holiday season, you'll have heard of the "Posadas" or "preposadas" a series of parties in the nine nights leading up to Christmas. In its most traditional form, the "preposada" commemorates the journey of Mary and Joseph as they sought shelter in Bethlehem before the birth of the baby Jesus. Each preposada follows a similar format and choreography. The key moment is a procession where participants reenact the journey of Mary and Joseph. Sometimes the partygoers will even dress up as various characters, including angels and shepherds, and lead by a couple symbolically dressed as Mary and Joseph.
Before the procession the participants will usely pray a rosary.
Mary and Joseph then walk a course that could involve different houses or more simply just go around different spaces in a church while singing Christmas carols (villancicos). At each house, the participants ask for shelter, recreating the biblical story of Mary and Joseph being denied shelter (posada) by different innkeepers. Eventually, one home (pre-arranged) opens its doors to the procession, symbolizing the inn that eventually let Mary and Joseph stay in its manger.
After the procession, there is often a celebration with food, drinks, and music. Traditional foods served during Las Posadas include tamales, buñuelos, atole, and ponche navideño (a warm fruit punch). All the food is provided by the community and in its modern version local businesses some of the food is provided by local busineses. Evenutally the children in the group will break a piñata, which is typically a seven-pointed star representing the seven deadly sins. The act of breaking the piñata symbolizes the triumph of virtue over sin.
Ironically, the most traditional form of posada is celebrated more outside of Mexico than inside it. The religous celebration has evolved into hardcore partying and debauchery in major cities like Mexico City. Smaller towns in Mexico and especially immigrant communities in the US preserve the more traditional religious form of the celebration, so you're more likely to find traditional posadas in Los Angeles than in Cancun!