Punyahavachanam (often referred to as Punyajanam) is a Vedic ritual performed to purify a person, home, or object. It is most commonly conducted on the 11th day after a child's birth or a death in the family to remove ritual impurities. Harivara.com Punyahavachanam Mantra Excerpt (Tamil Lyrics)

The exact string of syllables varies by lineage. However, the most widely accepted version of the Punyajanam Mantra in Tamil is a blend of Sanskrit and Tamil, often dedicated to Lord Shiva or Lord Murugan.

Om Punyajananaaya Vidmahe Shivapurushaaya Dhimahi Tanno Janam Prachodayaat

The Best Time (Muhurtham)

I think you're looking for the "Punyajanam" mantra in Tamil, which is a sacred Hindu ritual. Here's some information: