Have active Sims wear the same festival-appropriate clothing as other attendees

Wndblwz​ I do this already for played Sims with Formal/Party outfit categories. As we're allowed 5 per category I set Festival outfits in one or both of those. I don't use the first or second slots and I've never seen my previous households turn up in the wrong one to a regular Formal event when I'm not playing them (and bitterly regretted a cosplay outfit) so it works for me. I do this for the Romance Festival, Festival of Light (where outfits can be bought), Geekcon for example. It depends on the Sim's interests and which festivals they would be likely to attend. I set them for kids too.

It's more difficult with unplayed Sims but you can modify Townies outfits with the cas.fulleditmode cheat.

Your approach is probably the only solution right now. I generally don't think ahead but that's what it is going to take.  :) 

Cheats are so helpful in this game...they almost function better then toggles.

Wndblwz​ I just got the chance to test this in a couple of different settings, I used the "Ask to see outfit" social at a festival, picked their Party outfit and the Sim that my Sim was chatting with changed into it and stayed dressed that way. I couldn't remember if that social caused the Sim to keep wearing the outfit that my Sim asked to see or if they'd only change for a minute. So that could at least work in a social group if say you gave them all a crow outfit for example and travelled as a group then got your Sim to "Ask to see outfit" for each of their travel group. It would be tricky if not in a social group because you would have had to add the outfit for random townies and then befriend them and that would be a lot of work. 

simsplayer818​ I appreciate your testing this!  I use the "Ask to see outfit" social but what I want is my Sim to be dressed in the festival-appropriate theme... It seems NPCs always show up dressed according to the theme. Now I'm just using your suggestion to have the festival attire in my Sim's wardrobe.