Pradosh is thirteenth day of the fortnight with presiding deity Sri Kaamdev,
Twelfth day of the fortnight is ruled by Lord Vishnu,
Fourteenth day of the fortnight is ruled by Lord Shiva,
Thirteenth day placed in between twelfth and fourteenth day, with a flux in between which revoke the truth of immunity in life,
Presiding deity of thirteenth day Kaam-Dev is ruled by Shiva, hence the truth of Thirteenth day is ruled by Shiva in comprehensive way,
May Lord Bless all,
Thanks please