If they send you an email, where whatever that email tells you. There's no need to wear a suit if they BC. And feel free to ask them, for God's sake. Recruiting is not full of evil people who will snicker at questions like that.
My firm was BC. I showed in BC the first day. It was orientation. Thank God I did not wear a suit for no reason. Only 2 people out of nearly 30 did. I kept a suit in the office. I wore it once in the span of the next 60 days, and that was only on the last day when I decided to take it home.
They told us they didn't care what we wore on the first day, but that the dress code was BC/feel free to wear that (plus, IIRC, it was hot as FUARK in NYC in late May/early June). Moreover, IIRC, about half to 2/3 wore BC on the first day.
Finally, on that day, no shits were given about what anyone wore. And everyone got an offer.
Also, it's a good idea to keep a few jackets/ties in the office in case something comes up. I wore BC every day, but had a suit/tie combo in my office closet. Good thing, cuz on a few occasions I was offered a chance to sit in on depos/client meetings and didn't have to say, "um sry I haz no coat and tie lol."