Jewish Holidays

In every generation, a person has to see himself as if he left Egypt. The Seder divides into four parts. the four parts correspond to the four cups.

The first Mitzvah Am Yisrael received was the Mitzvah of Rosh Chodesh. This was Hashem's first communication with the nation of Israel. What is so important in the Mitzvah of Rosh Chodesh? How can it be that the year starts in TIshrei, but the first month is Nissan?