Elio doesn't celebrate Halloween. He used to as a kid, but he outgrew the holiday as he got older. So his "ideal Halloween" is taking part of other festivities or just stay at home and chill for the evening; it's another run-of-the-mill day for him.

However, if asked to attend a cosplay party on Halloween - and depending on who sent out the invitation, he may reconsider. He's a socialite after all.