Generally, Rikta, Chaturthi, Navami, Chaturdashi, Saptami, Ashtami, Bhadra, Purnima, Amavasya, and Pratipada Tithi are inauspicious. These should not be selected for starting an auspicious event or work. But if Ashtami falls on a Tuesday, it is known as Jaya Tithi, which is good for victory. This is an exception to the rule.
Inauspicious Muhurat, 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.
One should perform Snan-Daan, take a sacred bath and do charity to the needy persons on these Snan-Daan Tithi.
Ashtami Tithi, an Inauspicious Muhurat
Most of the Tithi create Yoga when joining 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, and perform Puja and Havan or Homa on this sacred Tithi, every month. Generally, Ashtami is also avoided for performing auspicious events.
Rikta Tithi, an Inauspicious Muhurat
These three Tithi Chaturthi, Navami, and Chaturdashi are known as Rikta Tithi and are inauspicious. Rikta means empty. One should worship and take precautions during these Tithi to avoid obstacles or delays in their work.