Not long ago, the Day of the Dead festival was a mystery to most Americans, and even some Mexicans, too. People who did know about it tended to think it was the “Mexican Halloween,” which is not true. But in recent decades, this annual observance has spread from its origins mostly in southern Mexico to the rest of the country, and into parts of the United States as well. At first glance, the celebration’s emphasis on skeletons and graves might seem ghoulish.