I figured it’s some kind of vague “energy” stuff they can siphon off. We already know that (in this series) succubi don’t actually have sex with people, just show them lewd dreams; so I don’t think they’re going the uh, “bodily fluid” route with vitality either.
Special love\sex night around the world? Author, come on. It's only in Japan, for some unknown reason, Christmas became romantic event for couples. Well, Japan has tendency to modify western holidays in some awkward ways.