It's from the 4th century AKA pagan Roman tradition transformed into Roman Catholic feast day.

The Roman Catholics are big into worshipping the dead. Of course, they'll use the word "veneration" in place of "worship" and many Catholics have no idea first class relics are stuffed in the altar or what first class relics even are.

First class relics are body parts of dead "saints."

Look up Sedlec Ossuary if you want further proof of these claims.

And Sedlec Ossuary has its own website-

The Sedlec Ossuary is nothing spectacular in the outside. It is a small chapel located in Sedlec, in the suburbs of Kutna Hora, in the Czech Republic. You would think that it is just an average old medieval gothic church.

As you enter the Sedlec Ossuary though, you will soon realize why it is one of the most amazing and unique churches in the world.The Sedlec Ossuary is artistically decorated by more than 40.000 human skeletons.

Thus it is also known as the Church of Bones or as the Bone Church.

One of the most fascinating artistic works inside the Sedlec Ossuary is the big chandelier of bones that lies in the center of the Church of Bones. The immense chandelier contains at least one of every human bone.

Another impressive artwork is the coat of arms of the Schwarzenberg family that is also made of human bones. While there are other macabre places to visit in Europe like the Paris Catacombe, the Sedlec Ossuary is really unique in nature.

https://sedlecossuary.com/