So, we've missed threads for these events:
[Event 129.3] Charity Wind Union Angel event
[Event 129.6] Light Tower event
[Event 130] Fleurety (Passionate Banquet) raid event
[Event 131] Ikkitousen collab advent, probably replaced by this event
With Kitty being MIA, nobody bothers to make threads for these anymore...

So then. We've probably lost the Ikkitousen collab. We might get it later, for example Haruhi was delayed by a lot but... ... Haruhi wasn't replaced by an original event. So you know. Don't keep any hopes.

But, let's talk about this event. It's an advent against five enemies: Phoenix, Heryshaf, Kyuki, Python and Marchosias. You'll probably mostly fight against Kyuki, since she's the one in Ultimate difficulty. Drops are about normal and the event lasts almost three weeks. So you have five times the farm and three times the time, have fun!

Now, let's talk about those things to farm here:
The Eidolons:
Marchosias is basically the only useful one. She's not only the only one with decent stats, but also has a good on-use (Fire really struggles with Burst cap). Veterans might keep her second copy, but the rest are just are just Eidolon Orb fodder.

The weapons:
Please notice that this advent drops its own unique materials, and therefore you can't find the materials to FLB weapons here. I will be noting when your next chance to farm those drops is, so hang on to the weapons until then.

Phoenix Bow is a great weapon to have. Bows are great. It will take about 11 weeks until Phoenix is in advent rotation, which isn't bad either for those who are lacking in mats.

Heryshaf Hammer is decent, but will be lacking FLB for over a year. Probably hold on to it, though, as Hammers are also great things. Who knows, maybe the FLB will give it triple-skill as powercreep when it eventually comes?

Kyuki Bow is also a great weapon to have. Bows are still great, after all. Sure, it's HP(++) instead of Atk(++), but still plenty good. Next chance to farm FLB mats is in only 3 weeks too.

Python Staff is... questionable. Being Vigor(++)/HP FLB (coming up in ~7 months), it can work but... it's rough. In general, Atk/Vigor/HP is what you want, not Vigor/HP alone. And you usually pick Atk/HP over Vigor/HP. Again, it can work, especially if you can guarantee yourself a MLB Dark Catastrophe Eidolon coming up a mere ~12 months from now as a P2W gacha Eidolon. Oh yeah, good luck with that!!

Marchosias Arcane is an interesting one. As a Exceed(++) with FLB, it will give you 50% burst cap by itself. So you just need two to reach full cap, without the help of Phaleg/Fire Catastrophe! But, there are two problems. One, it's not coming out until ~9 months from now. Two, the secondary skill it gains is Pride(). Now, that is decent, but while I consider Atk(++) and Pride(++) to be about equal, small Pride is a bit more lacking when compared to small Atk. Still, it is usable (as Exceed(++)/HP would've gone straight into the trash). As such, I do recommend that you save this thing, as it just might solve your Exceed problems much later down the line.

And lastly, there's a "new difficulty" which is a Nutaku original one. I don't think there is anything new there, as it looks like they just slapped the five bosses as Ragnarok difficulty in a row and called it a day. Veterans can just AAB that without any issues (waiting for videos of AB clear, I should've thought about that before doing my runs), it's not like Medusa is in there or something. The only "original" thing about this difficulty is that you try the fight a total of five times throughout the event. You should do it once for the free SSR ticket, but otherwise all that you gain from this fight is one advent core per run.