Kommentar zu Jirmejahu 1:12

You have seen well This almond tree hastens to blossom before all other trees. I, too, hasten to perform My word. And the Midrash Aggadah (Ecc. Rabbah 12:8) explains: An almond tree takes twenty-one days from its blossoming until it is completely ripe, as the number of days between the seventeenth of Tammuz, when the city was broken into, until the ninth of Ab, when the Temple was burnt.