Almost every ruined-hair story starts with a second chemical process done too soon.
Short answer: wash it a few times, wait at least a week if you can, and go to an experienced colourist for a correction. Do not put another chemical on your hair tonight.
Colour keeps settling for a few days after the appointment, and it usually looks less extreme after two or three washes. More importantly, two chemical processes back to back is what actually breaks hair. Broken hair cannot be fixed before a wedding. Wrong colour usually can.
Too orange or brassy — usually corrected with a toner. Gentle, quick, low risk. Too dark — hardest to fix and needs the most care. Removing colour is harsher than adding it. Patchy — a colourist can even it out, but it takes time in the chair. Book a long slot. Damaged and rough — this is not a colour problem any more. Treatments and a trim, and then plan the hairstyle around it.
Be honest with your stylist about the date. A good one will refuse a risky correction, and that refusal is worth listening to. A toner or a gloss is usually safe. A second bleach is not. Talk to your hairstylist about a style that hides the uneven parts. Braids and buns hide a lot.
Box colour at home over a salon colour. The most common way a fixable problem becomes an unfixable one. Bleaching again within days. This is what breaks hair. There is no undo. Washing with hot water repeatedly to fade it. It fades slowly and damages more than it helps.
No second chemical process tonight. Wash two or three times and look again. Go to an experienced colourist and tell them the wedding date.
At least a week if the wedding date allows. Colour settles and lightens over a few washes, and two chemical processes close together is what breaks hair.
Usually yes, with a toner, which is gentle and quick. Going lighter after a dark colour is much harder and riskier close to a wedding.