Proto-Indo-European *dʰeh₁s- was used in words for religious concepts, and was possibly an extension of *dʰeh₁- "to set, put". In Greek it became θεός theos (from earlier *thes-os) as in theology. ἔνθεος or ἔνθους was "inspired by god", and ἐνθουσιασμός was "inspiration, frenzy", whence enthusiasm.
The suffixed zero-grade form *dʰh₁s-no- became Latin fānum "temple" and fānāticus "belonging to a temple, inspired by a divinity" (Skeat).
The suffixed form *dʰeh₁s-to- became Latin fēstus "festive" and festa "festal ceremonies", and Italian festone, borrowed into English as festoon, as in "decoration for a feast".