On the day of Panguni Uthram, Thiagaraja (Lord Shiva) at Tiruvarur shows his Rudra Patham to the devotees. He shows vama patham on Margazhi Tiruvathirai star day and rudra patham on Uthram, Panguni day.
Normally Tamil months associated with a Star are observed as auspicious days which invariably is Pournami day also.
Chitra Pournami
Vaikasi Visakam - Pournami day
Aani Moolam
Aadi Pooram
Avani Avittam
Karthigai-Krithigai
Margazhi-Thiruvadirai
Thai Poosam
Masi Magam
Panguni Uthram
All the above days are full moon days and considered auspicious days.
Panguni Uthram is also happens to be birth day of Lord Ayyappan
The pournami day (full moon day) is always auspicious for the Gods
எண்ணாயிரமாண்டு யோகம் இருப்பினும் கண்ணார் அமுதனை கண்டரிவாரில்லை உள்நாடி ஒளிபெற உள்ளே நோக்கினார் கண்ணாடி போல கலந்து நின்றானே