On TE, the explosions scale from T-ATK, but the melee scales from S-ATK. And yes on ET, Wand scales from S-ATK.
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if i use S1:strike boost with 200 s-atk to get 5% damge. Does 5%damge apply on T damage?
On the wand affixes specifically Et scales with s-atk, but uses the affixes of whichever is higher to increase the damage, so if you want a t-atk wand to also use for te you can. That said, wands REALLY want the attack speed bonus from wand clobber, which is bordering on useless for etoile, so you might be better off using a seperate wand anyway. And yes the boost s1 SSAs increase all of your damage.
The official site is up, but doesn't have anything on the web panels yet, just the event schedule: http://pso2.jp/players/event/newyear2020/
Both that page and bumped should have more details next week when the event begins.
Just to make sure: Are S Add capsule drops affected by RDR?
One more thing. I can't quite remember, but are there affixes that boosts Soul success rate, Apprentice Soul in particular?