I meant to post this earlier this week but forgot. I always post the dates for major Buddhist holidays for the year in my first post for the New Year - here are the major Buddhist holidays for 2013, and a little about each: 2013 Buddhist Holidays
This is really just the tip of the iceberg, and each branch and even country has additional ones. The way these are celebrated are often very culturally specific.
As I've often said here, I'm not a big one for holidays, festivals, and rituals, but I respect that this is a big part of being a Buddhist for many people around the world. These days serve as 'markers' for people to focus on different aspects of their path, to stop and give thanks, and to share their practice with other practitioners. I do think this is a very powerful role that religions play in human life, and although for me Buddhism is not an organized religion in the traditional sense, for many people around the world, it is.
Hope you enjoy learning about these!