Rosh Hashanah Question

I have recently heard that the phrase “no one knows the day or the hour” is an old Hebrew idiom in reference to Rosh Hashanah. Can you explain? Thank you

Shalom!

Thank you for your question.

That passage has a number of interpretations. The interpretation closest to what you seek is the idea that no one knows when they are going to die, so we should live our lives performing mitzvot and good deeds. So too, we should always be in a state of repentance, always trying to make ourselves better.