It is believed that worshipping Lord Shiva at Pradosha day will relieve one from sins and help him attain Moksha. The Pradosha puja is considered to be more auspicious among all the Pujas performed by the devotees of Lord Shiva.
It is believed that during Pradosha period all universal beings and Gods come down to earth and attend the worship. The importance of observing Pradosha is mentioned in the Pradosha Mahatmiyam from Shiva Purana and hence Shiva devotees observe Pradosha Puja with sanctity. This bimonthly affair indicates the meeting of the Sun and the Moon in a horizontal line during their movement on their axis.