Shaykhul Islām Ibn Taymiyyah رحمه الله: "And he should strive in du'aa on the afternoon of ‘Arafah. For indeed, Iblees is not seen lowly and disgraced more than on the afternoon of 'Arafah due to what he sees of mercy descending." [Majmu'ul Fataawa V26/P131 | Translated by Abu Anwar an-Naijiriyy]