הלכות תפילין ה:יט
Klal Yisroel is compared to the moon, which constantly waxes and wanes. This comparison hints to the notion that we must constantly renew ourselves at all times. We must live every single day and moment as if it is completely new - and this certainly applies to one's avodas Hashem. We must never age, Heaven forbid, even if we live 1,000 years, because every day is considered completely new. All of the days of the the holy Klal Yisrael are like a month. That is, a new existence every moment, which is the crux of a long, eternal life.