Namaste to all,

I was reading a Ganapati Upanishad and there it says that one who recites this and fasts on the 4th day of the lunar month becomes scoolar. What i would like to know is about those lunar months? I don't understand it when they appear? Is there any one that knows something about that and it is ready to explain to me about that 4th lunar day? (When, How, Why,...)