G: I checked the current official Pokémon GO announcements. I’m not seeing an evolution window this week (Aug. 31–Sept. 6) where evolving ordinary Pokémon grants old Community Day/exclusive moves. The big event is GO Fest 2026: Mega Finale, Sept. 5–6, but its special attacks work differently. citeturn0search8turn0search2

What is happening:

Mega Finale: Saturday Sept. 5 and Sunday Sept. 6, 10:00 a.m.–6:00 p.m. local time.
Mega Chesnaught, Mega Delphox, and Mega Greninja get special additional Mega attacks (Seed Bomb+, Mystical Fire+, Surf+), but these are tied to Mega Evolution, not “evolve Quilladin/Braixen/Frogadier during these hours to get a legacy move.” citeturn0search2
Several other eligible Megas now get their own additional + attack while Mega Evolved as part of the new Mega system. Again, you don't need to hold ordinary evolutions for an evolution-move window. citeturn0search2

So if your actual question is “Should I avoid evolving anything right now because there’s a free legacy-move opportunity coming this weekend?” — apparently no.

There is a Gible Community Day Classic later in September, which is exactly the sort of event where I'd expect an evolution-exclusive attack window, but that's not this week. citeturn0news11

urlOfficial Mega Finale event pageturn0search8

G: Yes — good catch. Do NOT evolve that perfect Fennekin yet unless you've chosen the Fennekin path. My first search missed an important event-specific mechanic.

I explicitly searched Fennekin / Delphox / Blast Burn, and Blast Burn is available this week during Mega Ascension / GO Pass: Mega Finale.

The crucial restriction is:

If you select Fennekin in the branching Timed Research, Delphox evolved from Braixen during the event will know Blast Burn. The Fennekin-path Delphox encounter also gets Blast Burn. citeturn1search1turn1search2

The event runs Monday, August 31 through Sunday, September 6, 2026. citeturn1search2

So for your 100% Fennekin:

Fennekin → Braixen → Delphox this week, after choosing the Fennekin path = Blast Burn.

And importantly, the wording specifically says “Delphox evolved from Braixen”, so this isn't limited to the particular Fennekin supplied by the research. Your existing hundo qualifies, provided you've selected the Fennekin path. citeturn1search2

So you were absolutely right to make me search Blast Burn specifically before evolving the hundo. My previous answer's conclusion that there wasn't a legacy-move evolution opportunity this week was wrong.