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