So funny story. Those ARE her formal ceremony robes.
Acolytes and junior priestesses have to wear the standard robes for their duties (the white robe with the green mantle). Full priestesses use that outfit for day-to-day work (the mantle has a gold trim to show they're a full priestess but sometimes I forget to draw it), but they're allowed to wear their own designs for "formal occasions." Senior priestesses have no restrictions on robe design, and the High Priestess is supposed to be fancy 100% of the time.
When Konsi got promoted to full priestess, she had this set made for the ceremony, but hasn't had an opportunity to use the outfit since. It's the nicest outfit she owns, and she didn't want to do a "rush job" on something for the date.
Now, eagle eyed readers might think "wait, but she went to a tailor" - and she did, but she's not currently wearing the outfit she had designed (it's too complex for them to have made it in time) - however, that doesn't mean the outfit won't be making an appearance...