The average age of a human being is 120 years. Thirukadaiyur Shastiapthapoorthi is celebrated on the person completing his 60 years exactly on his birth star. This is because the person’s birth year, birth month, birth star at the time of his birth will reappear again only on his 60 years completion. Accordingly, Shastiapthapoorthi is celebrated on that day. Hence that particular day is considered auspicious for celebrating Shastiapthapoorthi, irrespective of Ashtami, Navami, Kari Naal, Tuesdays, Saturdays etc. Shastiapthapoorthi, thirukadaiyur 60th marriage and other ayush homam at thirukadaiyur is celebrated purely on the Jenma Nakshatra (Birth star) date.To note, the birth year, birth month and the birth star would appear for the third time at 120 years.
To know why the reason why Thirukadaiyur Shastiapthapoorthi and other ayush homam at thirukadaiyur is being celebrated, we should know the Temple History i.e, Stala Puraanam of Thirukadaiyur temple. The name of Shiva in Thirukadaiyur temple is Swami Amirthakadeshwarar. In all Shiva temples, Lord shiva occupies the form of Lingam called Shiva Linga Swaroopam. Let us know why Lord Shiva got the name of Amirthakadeshwar. From the Ocean (Paarkadal) for taking the Amirdham, there was war going on between the Asuras and Devas. Lord Vishnu took the form of a dazzling beauty (Mohini Avataaram) and with an intention that Devas should get the Amirdham, he cheated the Asuras and brought the Amirdha Kadam i.,e Amirtha pot to this Thirukadaiyur temple. The name of the pond behind the Thirukadaiyur temple is called Amirtha Pushkarani. Pushkarani means Thirukulam (Holy pond). So, Lord Vishnu decided to give the Amirtham to Devas in this Pushkarani. This avatharam name for Lord Vishu is called AmithaNarayana Perumal. This temple is located in the south of Thirukadaiyur with the God name as Amirthanarayana perumal and Goddess Amirthavalli.
Hence, before distributing the Amirtham, devas took bath (Snanam) and to their surprise when they returned the Amirtha kalasam was not there. This is because of the reason that Devas did not pray the first God Vinayaga (Lord Ganesha) before taking the Amirtham. So, Lord Vinayaga got angry and had stolen the Amirtha kalasam and hided the same. Hence, this temple Vinayagar is called as Kalla Vinayagar. In Sanskrit, he is known as Chora Ganapathy. So, every deity in this temple has a name derived from the story, like Amirtha narayana perumal, Kallan Vinayagar, etc. The place where the Amirtha kadam was hidden was a Vilvavanam (vilva forest) and hence this Kshetram was popularly known Vilvaaranya Kshetram. Since this Amirdham was hidden beneath the Vilva Tree, it had become Swayambhu Moorthy (Eswaran). Since the Amirdha Kadam has become Swayambhu Moorthy Eshwar, the Swami here is known as Amirthakadeshwarar i.,e Amirtham + Kadam + Eshwarar.