I'm off to a wedding next month with my baby who'll be 5 months and a very messy eater I'll still be breastfeeding, so what on earth do I wear!? I've only ever worn dresses to weddings - I'm not exactly a trouser suit type of person. I'll probably have to feed in the church and definitely a number of times at the reception.
A dress with a stretchy jersey wrap-type neck or v neck. Something like this. Accessorised with a pashmina worn to keep arms warm, then adjusted to protect your modesty when you whip a boob out in the church. Easy. I never bothered with specific breastfeeding clothes, just bought a few nice things that I would wear agin.