The month of Elul is a month of preparation for Rosh Hashanah and Yom Kippur. That’s why Jews blow the shofar (almost) every day of the month.

When to blow the Shofar?

The optimum shofar-blowing time is right after morning services, when everyone is still together. Missed it? You might still want to catch a shofar-blowing some time before sundown.1 We blow the shofar every day other than Shabbat, starting from Elul 1 and ending on Elul 28. We do not blow on Elul 29, the day before Rosh Hashanah.2

What to Blow?

Using a kosher ram’s horn, we blow a condensed version of the full sequence blown on Rosh Hashanah:

One long blast, three midsized blasts (with a little tiny blast), nine short blasts, one long blast.
One long blast, three midsized blasts (with a little tiny blast), one long blast.
One long blast, nine short blasts, one long blast.

That’s how it’s done in Chabad; there are others who just blow the first segment. On Rosh Hashanah, the sequence is much longer, with many more requirements and specifications.

Why Blow the Shofar?

For lots of reasons. Here are just a few:

a. After Israel sinned with the golden calf, Moses spent 40 days pleading for forgiveness. Then he ascended Mount Sinai once again for another 40 days—after which he descended with the second tablets. This ascent, which began on the first of Elul and lasted until Yom Kippur, was accompanied by shofar blasts. To commemorate this, we blow the shofar during the month of Elul.3

b. Elul is the month during which we search our souls in anticipation of the High Holidays. The soul-stirring shofar blasts inspire us to come closer to G‑d, as we read, “Shall a trumpet be blown in the city, and the people not be afraid?”4

c. Blowing the shofar—which is actually a Rosh Hashanah activity—for a month in advance confuses the prosecuting angel, who now has no idea what day is the real Rosh Hashanah.5

Huh? How is blowing the shofar for a month going to confuse the prosecuting angel? Nobody ever delivered a Jewish calendar to his door? Wouldn’t the crafty angel catch on after a few hundred years?

The Rebbe6 has a wonderful insight into this:

First of all, this isn’t the only time we’re out to befuddle the prosecution. On Rosh Hashanah we blow the shofar more than necessary, the Talmud tells us, “to confuse the prosecuting angel.” On that Talmudic passage, Rashi7 explains: When the prosecutor sees how we cherish G‑d’s commandments—going far beyond the strict requirements—he simply has nothing to say.

Something similar happens when we blow the shofar for an entire month before Rosh Hashanah. By doing so, inevitably we’ll feel remorse over past misdeeds and set ourselves upon a fresh new path. If so, the case is already sealed—and we won. G‑d has already inscribed us in the book of life for the coming year—even before Rosh Hashanah. This leaves the prosecutor confused. What’s left for him to do when the trial date finally arrives?

That’s the meaning of “not knowing what day is Rosh Hashanah”—he can no longer tell when the judgment occurs. Because we proactively took care of the whole thing on our own accord—sort of a backroom deal between us and G‑d.

This is also why we do not blow on the day before Rosh Hashanah: By that point we are so confident that G‑d has accepted our sincere repentance during the first 29 days that we do not even need to blow on the last day of the month.

And the prosecution is out of a job.