Wedding guest, and the bag nobody plans for
The dress gets months of thought. The accessories get bought the day before, which is the wrong way round.
Built on: The dress you have already chosen.
Everyone plans the dress and nobody plans the bag, which is how so many wedding photographs contain an excellent outfit and an everyday handbag. The bag is the item to solve first, because it is the one with a hard constraint: it must be small, it must be carryable while holding a drink, and it must not be the black tote you use for work.
Small is smaller than people expect. Phone, card, lipstick, and that is the capacity of a proper occasion bag. Anything more goes in the car or with someone's coat. A wristlet strap is worth looking for specifically, because a clutch with no strap has to be put down, and a bag put down at a wedding is a bag that stays there.
On jewellery, the useful rule is to decide what the dress is already doing. A plain dress can take large earrings and a necklace. A dress with any detail at the neckline — beading, a collar, a ruffle, a strong pattern — should take earrings only. Adding a necklace to a busy neckline is the single most common occasion-dressing mistake and it is entirely avoidable.
Comfort deserves one line, since this is a fourteen-hour outfit. Heavy earrings at 2pm are a different proposition at 11pm. Under 10g each.