Origin
Día de Los Muertos originates from the Aztecs 3,000 years ago. The Aztecs believed that there was life after death due to the soul existing in the after life. As a result of this belief, Aztecs would celebrate their deceased ancestors along with the goddess of death for the entire 9th month of their calendar.
After the Aztecs were conquered by the Spaniards in the early 1500's, they pushed the Aztecs to adopt Catholic beliefs with a large one being how they viewed death. While not being able to eliminate the Aztec traditions they did heavily influence them. The largest impact was reducing the length of the celebration from one month to only 2 days. These days aligned with the Catholic holidays All Saint's Day and All Soul's Day
On the left is an an All Saints Day celebration and on the right is an All Souls Day celebration. Like Día de Los Muertos, decorating the grave of a deceased loved one is considered an important tradition to show them respect and honor.