Pesach: Leaving the Gates of Impurity

Every year 30 days before Pesach Hashem "lifts His face" משא פנים to Yisroel in His great kindness, and He begins to remove their souls from the chambers of impurity little by little, one thirtieth each night so that by the night of biur chometz [destroying the leaven] every Jew who commited a sin is now standing at the doorway of the outer chamber...and on the night of Pesach they do not enter that chamber at all, and all of them are absolved and free men. Hashem in His blessed wisdom decreed that this should occur little by little over 30 days so that Sama'kel [the Satan] should not feel the pain and he shouldn't come to accuse.

Chesed L'Avraham 2:57, R' Avraham Azulai