According to Jim Recap: The Best Man
Danny (Dan Aykroyd) and Laraine (Laraine Newman) return to visit Jim and Cheryl with some news. They are getting married - and they ate all the banana bread. What's more, Danny wants Jim to be the best man in his wedding. Jim, of course, is happy to oblige, so long as he gets to throw the bachelor party. Which is where things get a little dicey.
Cheryl has convinced Laraine, who in turn has convinced Danny that a traditional bachelor party isn't the way to go. In other words, no booze and no broads. So Jim agrees that he will do a bachelor party without either, so long as they can do it, because he has to do something for Danny. But then they are at the coffee shop, which is about all they can do without booze or broads, and that's when they are struck with an idea.
Well, the idea comes to them. It's a bunch of guys who come in and grab coffee and then run out again - but they are in great spirits, and it's decided that they will follow them and try and add some life to this party. The folk singer and coffee shop just isn't doing it, and something has to happen!
It seems, however, that these guys are fired up because they need some energy. They are members of the Polar Bear Club, which means they are about to go for a dip in Lake Michigan - and yes, it really is 20 degrees outside. No wonder these guys went for coffee first. They also seem to be in a bit better shape than Jim, Danny, Andy and the rest of the band. But you have to give everyone credit - they dive right in.
Unfortunately it takes an ambulance to get them out again - at least Danny - because his body just about shuts down in the process. And the next day, when he's supposed to get married, he is still in bed, and Jim is up in the hotel room trying to get him out of bed while Cheryl tries to quiet a restless mob of well-armed wedding guests. That's when the doctor tells Jim that nothing is physically wrong with Danny, and Jim knows that Danny just has cold feet. Not physically cold feet from the water, but cold feet for getting married, and he has to give him the courage that Danny gave him years ago. So that's just what he does.
