Americans celebrate any festival as anyone else would: with decorations, traditional foods, and surrounding themselves with friends and family. My family, for example, has a tradition of opening one present per person on Christmas Eve. Another family might have their big family dinner on Christmas Eve and avoid all the heavy Christmas Day traffic.
However, that said, there are whole communities that will ban together and celebrate specific holidays together. A group of Indian Americans might get together and celebrate Holi together. A Muslim American population might observe Ramadan together. Irish Americans might come together to celebrate St. Patrick’s Day in a more traditional way. It all depends on who is in the area and what they decide they want to do for a specific holiday.