Ex. 12:2 says, “This month (Nisan, when we left Egypt) shall be to you the first of the months”. The sages tell us that this verse is not only the first commandment addressed to the people of Israel, but it tells us two things: to have a calendar, and to start the list of the months with Nisan (see Rash’s commentary on this verse and Gen. 1:1).
Why is the Hebrew word for a month chodesh? It comes from the word for new. Ibn Ezra tells us that when we look up at the sky we see that the moon becomes new again twelve times a year, which is why we have twelve months.
Where then do we get the word shanah for a year? Once again we quote Ibn Ezra: “The sun proceeds on its course for 365 and a quarter days and then returns a second time (shenit), which is why a year is shanah”. I have heard it said that sun is linguistically connected with shenit, but that theory is too simplistic for the etymological experts.
On another level, it is important for human beings to divide their years and lives into sections and to plan what they will endeavour to do with their time at each stage.
The classical Jewish attempt at subdividing life as a whole is found at the end of Pir’kei Avot chapter 5, which tells us where we can or should expect to be at the ages of 5, 10, 13 etc. Events often affect the orderly rhythm of life, and some people’s pace is faster or slower than others’, but it is good to have a yardstick.