Mid Autumn Festival (Moon Festival) takes place on the 15th day of the 8th month of the lunar calendar. That's today, September 30th, 2012!

for an overview of Mid Autumn Festival, follow the links here below. they'll take you to some helpful articles on the subject:

http://taoism.about.com/od/holidays/a/MidAutumn_Festival.htm
http://en.wikipedia.org/wiki/Moon_Festival

Important Festival Vocabulary:

中  zhōng - center, middle
秋 qiū - autumn, fall
节 - jié festival
中秋节 zhōng qiū jié - mid autumn festival
快乐 kuài lè - happy, joyous
中秋快乐 zhōng qiū kuài lè - happy mid autumn festival
月亮 yuè liàng - the moon
赏月 shǎng yuè - to admire the moon
月饼 yuè bǐng - moon cake