#drch4

rudy was KIA in the war, asgore's second in arms

vizier carol was highly concerned with chara's presence in the underground and thought she would bring disaster, growing further distant from the dreemurrs after her suspicions were confirmed with asriel's death

dess became a recluse after asriel was killed

the 3 of them moved out to snowdin sometime after perseverance was captured, not as far as toriel on account of her taking chara in being deemed foolishness, but still distant enough from asgore and his madness

Well, the thing is, that I don't necessarily need to imagine, because we did get a few details of the Holidays' existence in the Undertale Timeline.

In fact, the Holidays were first mentioned before Deltarune even released, as part of Undertale's Anniversary Winter-Themed Alarm Clock Dialogue Thingy, where Asgore reminisces about Rudy in detail.

Rudy seems to have been born in the Underground, as Asgore implies he has never seen the sun.

Asgore and Rudy met in Hotland University, similarly to how in Deltarune they were old college buddies, but here it's a bit... unclear what Asgore was doing in College. Like, Rudy is said to have 'suddenly fallen down' but Dess and Noelle are explicitly alive to see the Barrier fall.

(And 'get to grow up' seems to imply that Dess and Noelle are still fairly young.)

So the Holidays are not from the War of Humans and Monsters Era, and not from the Royal Siblings Tragedy Era, they are explicitly contemporary to the current-day of the Underground. So what was War Veteran Legendary King Asgore Eternally-Stuck-at-40-Years-Old-Cause-His-Kid-Died-a-100-years-ago-That's-Just-the-Lowest-Estimate doing at University? Was it, like, he came in to give a speech and started getting chummy with one of the students? Or was he just like "Well, now that the Underground finally has Higher Education, I guess it's about time I get my BA that sounds like fun?" We don't really have any clarification on that.

But it is pretty clear that the Holidays are around at the same time as the events of Undertale, they live in Snowdin (cause, I mean, obviously)

Rudy died very suddenly, presumably from the same illness that he's suffering from in the Deltarune timeline (The Underground has a mish-mash of fantastically-advanced and slightly outdated technology. Is it possible that they just didn't have the medical technology that is available in Hometown to diagnose his disease?) and that Dess and Noelle are both alive when the True Pacifist Ending happens.

Dess and Noelle are also alluded to in Sans' dialogue in the Exclusive Dog Shrine/Casino in the XBox Version of Undertale

Outside of once again reiterating that Dess and Noelle are alive in Snowdin at the present-day of Undertale, the only potentially-interesting detail is Sans describing them as "the antlered girl and her big sis"... which sounds like it's implicitly saying Dess doesn't have antlers, which is contradicted by Tenna's Flashback in Chapter 3

So either Dess started filing-down her own antlers as a sign of rebellion.... or this was just a slightly-awkward bit of dialogue for the sake of the audience understanding that this is about Noelle and Dess.

(I also think Dess and Noelle are likely to be the sisters from this set of Echo Flowers in the Wishing Room.

But I have to admit my evidence is rather circumstantial and vibe-based.)

Most of the details we have about the Holidays in the Undertale Timeline are rather... ordinary (even considering Rudy's friendship with the King of All Monsters and his tragic early death). I think it's a very deliberate sort of 'roleswap', that a lot of the major Monsters in Undertale have more secondary or background roles in the Deltarune Timeline. While the Holidays were basically just ordinary civilians in the background of the Undertale timeline and now they are extremely central to the plot of Deltarune.

But the most curious thing with the Undertale Holidays is Carol, or rather, Carol's absence. The only reason why know she probably exists is because Asgore mentions Rudy's marriage.

But otherwise he only ever talks about Rudy and his daughters. It's also notable that they live in Snowdin, when one of the most memorable pieces of Minor NPC Dialogue there starts with...

Everything we've seen of Carol so far seems to imply she's very serious about her job, an extremely competent leader and a take-charge kinda lady... It seems kinda weird of her to just let Papyrus become the unofficial authority of Snowdin Town. It really makes one wonder...

I've seen people speculate that Carol Deltarune's attitude is primarily a result of Dess' disappearance, that in the Undertale Timeline where Dess is fine, she's just more chill (...no pun intended) sort of person and maybe doesn't even have any interest in power or authority anymore. Or alternatively, that the trauma of Rudy's early and sudden death + the general hopelessness of being stuck in the Underground had drained her drive and passion, causing her to retreat from the world.

based on this post:

Also octagon is the worst shape anything has ever been and I may start crying next time I see one in the wild. 💥