Serving Bhagavān's Servants
Tatiyārādhanam (Tamil: tadīyar = those related to Him [Bhagavān's devotees] + ārādhanam = worship/honour) is the act of honouring Śrī Vaiṣṇava guests with food, care, and reverence. In the tradition, a bhāgavata (devotee of Bhagavān) is not merely a respected person but a manifestation of Bhagavān's own presence — serving them is literally serving the Lord.
Greater Than Serving Bhagavān?
Several Ācāryas teach that in the present Kali Yuga, tatiyārādhanam — serving the Lord's devotees — is even more meritorious than directly serving the Lord in the temple, because Bhagavān is more pleased when His own devotees are honoured. Piḷḷai Lokācārya discusses this in Śrīvacana Bhūṣaṇam.
Ponnadi Sāttrudal
Ponnadi sāttrudal (inviting a Śrī Vaiṣṇava to one's home) is the initiating act that leads to tatiyārādhanam. Receiving a devotee as a guest and serving them prasādam is a sacred opportunity, not merely a social obligation.