Imagine all the pagan holidays inscribed on a great scroll, and the Catholic Church in need of converts. What they did was pretty clever: they overwrote their own stuff (making a palimpsest), called the new holidays holy days (the syncretism part), made them obligatory (“holy days of obligation”), and miraculously, centuries later, it worked.
One of their early problems was the spooky Lemuria festival that ended May 13. The lemures (from which the nocturnal scary-looking lemur got its name) have been wandering around the Roman master’s house for three nights now, picking up his beans. Some of them were lares (good ghosts), but those larvae (bad ghosts) may have been just a bit too pagan – not to mention the bean part.
So Pope Boniface had a good idea – on May 13, early in the 7th century, he consecrated the Roman Pantheon to Mary and all the martyr saints. Voila: All Saints’ Day.
Two centuries later, however, not too many people had even heard of the Lemuria, but Samhein (think Halloween) had become really pagan. Voila: All Saints’ Day moves to November 1, and a new syncretic palimpsest is born.