In 28 days the Moon goes through it phases, Their is a New Moon every 28 days, and a full moon every 28 days, they are 14 days apart from each other.
Going from New Moon to Full, the moon is "Waxing".
When it is going from Full Moon to New(No Moon) it is "Waning".
Are you thinking "Blue Moon"?
http://en.wikipedia.org/wiki/Blue_moon