The Maharsha says in Bechoros (8b) that the 21 days starting with Shiva Asar B’Tammuz and ending on Tisha B’Av correspond to the 21 days from Rosh HaShanah until Hoshana Rabba. Rav Tzvi Meir Zilberberg says that we see from here that there is special Kedusha in these three weeks similar to the days of Elul and Tishrei. Moreover, our avoda during these three weeks will impact not only our Elul, but also our whole next year. He brings from the Toldos Adam that whoever strengthens himself in Avodas Hashem during the three weeks will merit to be get through the Din of Rosh HaShanah unblemished.
Similarly, he brings from the Oheiv Yisroel of Apta that these 21 days correspond to the 21 special days of the year including, the day of Shabbos, the day of Rosh Chodesh, seven days of Pesach, Eight days of Succos, Shavuos, two days Rosh HaShana, and Yom Kippur. They don’t simply correspond, says Rav Tzvi Meir. The three weeks actually encompass all the Kedusha of those days.
Although today these days appear bleak and lifeless, be”H with the coming of Moshiach these days will become festive days, climaxing on Tisha B’Av which is similar to Simchas Torah. Until then, the light of these days is hidden, yet we must still take advantage and tap in to the special Kedusha, says Rav Tzvi Meir.