I'VE BEEN SLEEPING ON THIS AND THEN WROTE A NOVEL OF ENTIRELY HEADCANON BS

So the First doesn't... have, any festivals. Not that we've been let in on anyway, and if the shock and awe at the concept of Ryne's festival at the end of the Eden quests is any indication, I'd say that people haven't really had it in them to celebrate much of anything for about ever since the Flood. This makes it very hard to do market research because we have no indications of what kinds of festival might be customary, but WORLDBUILDING IS MY PASSION so!!! here's my Thoughts:

-Kholusia before Eulmore happened was largely made up of fishing and farming communities with a heavy lean on hops, wheat and ale brewing. while there'd almost certainly be season-based harvest festivals and gatherings to celebrate the migration of specific fish into the local rivers, I imagine the most popular, highly anticipated celebration of the year would be something more collaborative with everyone living in the region- considering the local dwarves, the brewer's culture before the Flood, and the German inspiration of Novrandt, I like to think there was a very beloved beer festival much akin to Oktoberfest! something where the dwarves and people living in the lowlands can gather together and have brewing competitions, cookouts with the season's vegetables and best fish, and much-needed trade establishments for fertilizer, Talos hearts and materials, brewing machinery and other goods each community needs from the others.

Considering he is a chronic Beer Enjoyer and thrives in tavern hangout style socialization, as well as the fact that he and Lamitt both grew up in the area, I can only imagine that the local holidays would be the home of a lot of very comfortable memories- the baby steps to seeing the world as something bigger than an island he'd only just escaped.

-Dwarves, on the other hand, are very focused in on invention and tech, with a heavy lean on tradition keeping any sort of celebration sure to last through the generations. I can absolutely imagine there being anniversary celebrations of particularly renowned inventors, science fair style festivals, mining competitions, etc- ruled, collectively, by a very strict set of rules and rituals followed to the letter. Maybe not quite as inviting to outsiders, these ones, but something Ardbert would have been aware of simply out of proximity (the sound of heavy machinery travels far) and through Lamitt's own stories, having been banned from ever participating again.

-Voeburt!!! is a whole kingdom!!! you better BET they'd have holidays! Considering they're a mirror of Ishgard, I think it'd be Very Funny if they had Ishgard's holidays, minus the religion- something akin to Starlight, something akin to Valention's day, something akin to Hatching Tide (the latter of which I want to say for personal reasons used to take place in Wolkendorf because it's cute and the lore doesn't exist so it's my lore now). Nothing the same, for sure, but the general idea of "we have a holiday in the winter to bring a little warmth to the frigid cold" and "this is marriage season so here's our Love Holiday" and of course "spring rules: holiday edition". Ardbert seems like the kind of person who'd really like winter holidays, and considering his fond reminiscence on winters in the city, he'd certainly have been there for a few of them.

and as for a favorite- man, I can't think of him and his fondness without always thinking back to his talk about the cold of Voeburt and the way he'd loved the people there. Voeburt was a huge kingdom with entirely different ways and people, far from the tiny towns he'd once called home, and somewhere he went with the full collection of friends he'd found in his travels- what warmer memories, than holidays spent with the people you love the most, sheltering and warm despite the cold? I don't think he's the kind of person to make a huge big deal out of celebrations- his holiday party equates to being with his friends (who he already lived with, so that's automatically going to happen), drinking, probably stupid drinking games, gift giving, and attempting (attempting) to see some holiday sights without getting noticed/dragged into things. Easy enough at first, ninja training as time went on and people started looking for them/recognizing them more. :,) It's nothing he can replicate- his holidays, like his concept of home, had nothing to do with particular habits or places- it was the people, and the people are gone.

Nowadays, Starlight is... bittersweet in its familiar nostalgia. Important in a whole new way, after Endwalker- Roku (our canon's WoL) dying briefly after the Endsinger fight left Ardbert a whole new sort of drained and weak, and it was around Starlight that he was able to finally recover enough to drag his aching soul shreds over to find Olyzas and confirm he had not, in fact, been swept away in the Aetherial Sea. Neither of them had much energy for anything but looking out windows for the holiday, but it marked something special.