It's the opposite of a roast. While people in a roasting event would gather up to make fun of a single person, a
buttering event has people gather up to adore, praise, and
talk good about that person. Sometimes, it's extended to include everyone, so that each person has a chance to receive a variety of compliments. You can't talk bad about someone at a buttering event. The worst you can get to talking bad about someone is
constructive criticism, guidence, and maybe a compliment sandwhich.