It states (Shemos 12), לכם הזה החודש, "Rosh chodesh shall be for you..." It is up to the Jewish nation to decide when Rosh chodesh will be, which reveals the greatness of the Jewish nation.
The Midrash (Shemos Rabba 15:2) states, "The malachim ask Hakadosh Baruch Hu, 'When will it be yom tov?'... Hashem replies, 'Me and you will agree with whatever the Jewish nation decides. It is up to them whether there will be a leap year or not... Hakadosh Baruch Hu says to Yisrael, 'Before, I had the strength... but from now on, it's in your hands. When you say yes, it's yes; when you say no, it's no. No matter what, לכם הזה החדש, establishing rosh chodesh is up to you. If you want to establish a leap year, I will agree with you.'"
The Chidushei HaRim zt'l was ecstatic with this Midrash, and he would say, "The power is in our hands! When we say yes, it is yes; when we say no, it is no. Everything is up to us! If we knew our powers, we wouldn’t waste a moment!" If we understood how our decisions and actions affect the entire world, we wouldn't take away from the time we have to serve Hashem and daven to Him.
We can be compared to a king who runs a country but doesn't realize he is king. Everything was given over to us, and we just have to take advantage of that knowledge and reality.
We say in Ashrei (Tehillim 145:12), לבני להודיע גבורותיו האדם. The Yesod v'Shoresh HaAvodah zt’l says גבורותיו refers to the strength and greatness of the Jewish nation. The pasuk is saying, גבורותיו האדם לבני להודיע, "reveal to people their greatness." Let them know what they can accomplish with their Torah, Tefillah, and Mitzvos. Let them know how powerful they are.
