I'm 99% certain that speaking to Sellen about needing to defeat Radahn is the trigger for the festival, NOT telling Blaidd about it. Either that, or there is a counter and speaking to at least 2 NPC's about the festival/defeating Radahn causes it to start (e.g.: Sellen & Alexander)

The Radahn Festival is ACTIVE for me, but I have yet to meet any of the above requirements. Here's what I have/have not done (most current version, 12/09/22):

1. HAVE defeated Magma Wyrm and activated both its and Dectus Lift's sites of grace, but have NOT taken either lift nor entered Altus Plateau

2. Have NOT spoken to/met Jerren at all

3. HAVE discussed needing to defeat Radahn with Sellen, but have NOT told Blaidd or Iji or anyone else about it

4. Have NOT spoken to anyone about the festival other than Alexander (spoke to him 2x, Limgrave & Gael Tunnel)

5. HAVE spoken to/exhausted all dialogue with all available NPC's (other than those mentioned above) and completed all available content prior to entering Altus Plateau or continuing Ranni's questline past the point mentioned, EXCEPT for most of Greyoll's Dragonbarrow and pre-festival Redmane Castle (have NOT killed/angered any NPC's, though some have died on their own according to their script)

for some reason it always triggered just after beating rennala without going to atlus platau or even speaking to ranni, why tf did this happen?

Already triggered the festival, but Jerren is nowhere to be found. Anyone has a clue?

Couldn't they have made the festival only trigger AFTER you kill the duo? Replaying the game and was really confused because I already met the requirements so the entire place was empty and the doors were closed

I like that there is one major boss that can be fought without a dungeon.

How do you reach the giant in the tower tho

So the 2 bosses (misbegotten and knight) are gone if the festival is active? I have NEVER reached this castle without the festival active. :/

With no festival, there's a Redmane Knight on the rooftop of the plaza grace, he's wielding a greatsword imbued with flame (even when he's idle). I wasted 2 hours to get the weapon drop, only to find that it's just a common Knight Greatsword, and no flame whatsoever, bummer.

If you've activated the festival but need to get the enemies back for the items then do Sellen's questline and when Jerren leaves the enemies will return.