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,...)

Pranam,
Dani