The thirteenth day of the lunar calendar both waxing and waning moons is called Trayodashi. 1.5 hrs on either side of sunset time is considered auspicious. It is said the samudra manthan happened on this day and the first thing that came was the poison. Lord Shiva drank the poison and stored it in his throat. Dosha is our karma and Pradosha means to eliminate or get over karma. So any Puja / Meditation/ Sadhana done during that time will help us eliminate all our karmas. So especially, Shiva/ Rudra Puja done is very very auspicious. If this falls on a Saturday, it is called Shani Pradosha. Shani represents our past. So it very very auspicious for one who is undergoing shani period/ Sade saati etc.
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