Singling out the first third of Ramadan to make Du`a for mercy, the second third to make Du`a for forgiveness and the last third to make Du`a for ransom from Hell is an innovation for which there is no basis in Islamic teachings. There is also no justification for singling out these times for these Du`as, because all the days of Ramadan are equal in that regard. Rather the Muslim may make Du`a for whatever he wants of goodness in this world and the hereafter throughout Ramadan; that includes asking Allah for mercy, forgiveness, ransom from the Fire and admission to Paradise.

There is nothing wrong with repeating this dhikr – “Subhan alladhi yusabbih ur-ra‘du bi hamdihi wa’l-mala’ikatu min kheefatihi (Glory be to the One Whom the thunder glorifies and praises, and the angels [also glorify Him] out of fear and awe of Him” – when hearing thunder more than once.