because it is a proper noun...

it's the rule of thumb, actually. All proper nouns should be written with a capital letter. I'm just a bit curious about chinese calendar which when written in english, go with capital letters when in fact they are 'common nouns': "Year of the Metal Tiger" for instance. :)

A noun is a person, place, or thing. A proper noun is a noun that is also a name. Proper nouns are usually capitalized in English. They could be names of people (Susan, Mike, George), names of places (Beijing, Nile, Russia, Johnstown), or names of things (Apple Computer, Saturday).

Because...that's the rule!

Because the Rules of Capitalization said so.