If it's a gala you are talking about where truly formal wear is required (i.e. a long gown) I don't have anything and would have to go shopping. I did have a thrifted skirt (still in my donate bag, come to think of it) but honestly, the waist was snug when I bought it, almost an inch ago.. And I only wore it to one costume party. So I think it's safe to let it go.
In my special occasion capsule I have:
- Sleeveless bright navy sheath dress, quite body con. (Nicole Miller, like Angie's). Bought on super sale at Nordstrom's. $150 approx. This is what passes for a sexy dress on me. Shows arms, is knee length. snug.
- David Meister sleeved soft navy sheath. Not as body con or "sexy", a bit more conservative, not quite as dressy as #1; can be dressed down a bit. Also purchased on super sale at Nordstrom's. Approx. $150.
- Sleeveless Kamalikulture black & cream bottle bottoms ruched sheath. Not as dressy as #1; packs well; can be dressed down and de-bombshelled. (Purchased on sale for about $50.)
- Sparkly brocade multicoloured shell top -- thrifted.
- Halogen striped skirt. Crosses over with less dressy wear.
- Black jacquard kimono jacket, vintage Biba, bought on consignment.
- Black short tuxedo style jacket with reverse lapels.
- Navy tuxedo jacket. (But no pants to go with).
Okala ankle straps. Burgundy patent pumps. Gold sling backs.
Several inherited clutches -- silver, gold, pearl, black.
It's not perfect, and not completely functional. (I could used some bottoms for the brocade shell and some tops for the striped Halogen if I want to wear them to "straight" dressy events vs. arty events where I'll pair the brocade top with jeans and the striped full skirt with a pullover). But I find I can do most of my (admittedly few) formal events in one of these.
The sleeveless sheath takes me to: fancy dinner out; spring/ summer/ early fall theatre outings or opera (with topper for cooler temps); awards banquet; cocktail party.
The sleeved sheath takes (or could take) me to: winter wedding; winter memorial or funeral; winter dinner out; or, in the unlikely event that I'd need to do this, some sort of corporate/ businessy party.
The Kamalikulture sleeveless travels well and takes me to: work parties/ dinners out of town (with topper if needed); cocktail party; date night. With topper it has also taken me to a memorial.
Brocade top can be paired with a black pencil skirt for dinner out. Usually paired with denim for an "arty" dressed up look. Add the tux jacket.
Halogen skirt: Could take me to a holiday party, birthday party, dinner out.
This is long winded. If you need to edit, I would make a list of the types of events you've actually had in the past couple of years, and those you can anticipate. Then consider probable weather conditions. And go from there.
I think my "perfect" capsule for special occasion would include (in addition to what I have mentioned there, or upgrades to same) --
One long skirt and/ or gown
One sequinned shell
One lace pencil skirt
One organza or similar blouse
One tuxedo suit.
Oh - a couple of options for you to look at....different levels of dressiness and I deliberately kept them long and kept the super expensive ones out of this. There were some beautiful Tadashi ones, too.