Fondue is a winter dish. If you have a hankering for some melted cheese in the summer, eat raclette instead (raclette cheese melted and scraped onto baby potatoes and cornichons). Fondue is also a social dish. Never eat fondue alone, that would be even more depressing than solitary drinking. Even sharing it as a couple isn't quite good enough. This dish is meant to be shared with as many friends and family as can sit around the table and reach into the pot.
This combination of winter food and social gathering is what makes this a holiday dish, even though it is not specifically for Christmas.