Zmanim · The calendar and the moon

What is the connection between the moon and the Hebrew month?

A calendar you can see in the sky

The Jewish month is not an abstraction on a page; it is something you can lift your eyes and watch. It begins when the moon is reborn as a thin sliver, grows fuller night by night, and reaches its fullness in the middle of the month. So a festival dated to the fifteenth, like Passover in Nisan or Sukkot in Tishrei, is a festival kept under a full moon. The date and the sky agree. This is a people whose sacred time is written in light.

Where the tradition says it

The Torah gives this to us at the very threshold of freedom. Before the Exodus, Hashem tells Moses and Aaron: הַחֹדֶשׁ הַזֶּה לָכֶם רֹאשׁ חֳדָשִׁים — this month shall be for you the head of the months (Exodus 12:2). The word for month, chodesh, shares its root with chadash, new: the month is the moon made new. The Talmud, in tractate Rosh Hashanah, works out how that renewal is established and sanctified. And each month the tradition invites us to bless the renewed moon aloud in kiddush levanah (the blessing over the new moon), thanking the One who renews the months.

What to do

Learn the shape of the month by watching it. On the night the new moon first appears, notice the thin crescent and know a Hebrew month has begun; around the fifteenth, look for the full moon and remember that a festival sits there. When you can, say kiddush levanah under an open sky. Counting by the moon is a mitzvah our people has kept across the generations — you are one more link watching the same light.

Source: Exodus 12:2; Talmud, Tractate Rosh Hashanah..