A festoon is a decorative string of flowers that you drape across a room, dropping in curves between support points. If you're decorating for Halloween, why not try a creatively spooky festoon of dried roses covered in cobwebs?
Festoon probably comes from the Italian word festone, for "feast," and if you can imagine an Italian courtyard strung up with flowers and lanterns, then you have a sense of what it means to festoon a place. A noun or a verb, festoon can mean the decoration itself, or the work of putting it up. Festoons don't strictly have to be made of flowers, though they usually are. You can festoon with fabric too, or paper chains. Or cobwebs.