Look upward and see the almost-full moon. When the moon becomes completely full, you’ll know it’s the 15th of the current Jewish month. Right now we’re in the month of Heshvan.
Or, just remember that in the Jewish calendar the new moon is always the new month. In fact the Hebrew noun for “month” is HOdesh, and the Hebrew adjective for “new” is haDASH. Their shared linguistic root is perfectly obvious.
The new moon/month is always occasion for a Jewish holiday. It’s called Rosh Hodesh in Hebrew, literally “head of the month.” (Recall that Rosh haShanah means “head of the year.”)
Paradoxically, the English word “month” comes from the English word “moon,” and yet they are not (any longer) tied together. Can you look up at the sky and be able to tell that it’s October 18?
Learn Hebrew with Natasha Shabat 