Generally Rikhta, Chaturthi, Navami, Chaturdashi, Saptami,Ashtami, Bhadra, Purnima, Amavasya and Pratipada Tithi are inauspicious, should not be selected for starting an auspicious event or work. But if Ashtami falls on a Tuesday, it is known as Jaya Tithi and is good for victory.This is an exception to the rule.
Inauspicious Tithi of Panchanga
There are some inauspicious Tithi we find in Panchanga, which we should avoid for important auspicious ceremonies or events. Generally Purnima (full Moon day) Amavasya (new Moon day) and Pratipada or Padwa tithi (new moon day)) are considered inauspicious and one should perform Snan-daan, take sacred bath and do charity to the needy persons on these Snan-daan Tithi.
Most of the Tithis create Yoga when join some specific var(days) like Jaya Tithi which is formed due to the combination of Ashtami and Tuesday.
Ashtami is a Tithi, we observe Vrat, perform Puja and Havan or Homa on this sacred Tithi, every month. Generally, Ashtami is also avoided for performing auspicious events.
These three Tithis Chaturthi, Navami, Chaturdashi are known as Rikta Tithi are inauspicious. Rikta means empty. One should worship and take precaution during these Tithis to avoid obstacles or delays in their their work.