Days of Forgiveness and Atonement

The Maharsha (Chidushei Agados on Bechoros 8B) writes that just like the 21 days from Rosh Hashanah until Hoshana Rabbah are a time to atone for sin, so too the 21 days between the 17th of Tamuz and Tisha B’Av are a time when sins are forgiven because suffering and pain atones for transgressions (Sanhedrin 37B).