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Why do we not blow the שׁוֹפר on ערב ראשׁ השׁנה?
#1 We want to separate between the תקיעוֹת רשׁוּת, those which are optional (i.e. for the month of אלוּל which only have their source in מנהג) and the תּקיעוֹת חוֹבה, those which are required by the תוֹרה (and blown on ראשׁ השׁנה). Or #2, because we want to mix up the שָטָן (the accusing angel).