Dia de los Muertos is a traditional Meso-American holiday dedicated to the ancestors; it honors both death and the cycle of life. In Mexico, neighbors gather in local cemeteries to share food, music, and fun with their extended community, both living and departed. The celebration acknowledges that we still have a relationship with our ancestors and loved ones that have passed away.
In San Francisco, Day of the Dead has been celebrated in the Mission district since the early 70s with art, music, performances and a walking procession, which help us contemplate our existence and mortality -- a moment to remember deceased friends and family, and our connections beyond our immediate concerns. (from here)
Read more here about the history of the Day of the Dead celebration.