Caitra Sukla Purnima or Chithirai Pournami is among the more important full moon events of the year–because it effectively opens the year. It is “rtumukham” or the mouth of the seasons.
ऋतुमुखं वैषा पौर्णमासी संवत्सरस्य, या चैत्री पौर्णमासीति।
says the Mimamsasutra of Jaimini (MS 6.5.31)
Also in the Sabara-Bhasya, Sabara-bhasyam of Jamini, the earliest surviving and most authoritative commentary on the Purva-Mimamsa Sutras/ system of Vedic hermeneutics. (This specific passage is on the timing of the diksa for the Gavamayana, a year-long sacrificial session. There is a lack of specification (avisesa). When a scriptural injunction mentions a "full moon" without specifying a particular month, the Mimamsa method initially suggests an aniyama (non-restriction), meaning any full moon could theoretically suffice, before specifying Chithirai pournami.
We missed the event at Kannika Parameswari, which we learn about later – but this has nothing to do with Chithirai as the mouth of the seasons. It has more to do with Chitragupta–about whom, the Garuda Purana says:
तत्सर्वं च परिज्ञाय चित्रगुप्तो निवेदयेत् । चित्रगुप्तस्ततः सर्वं कर्म तस्मै वदत्यथ ॥
tat sarvam ca parijnaya citragupto nivedayet | citraguptas tatah sarvam karma tasmai vadaty atha ||
“Having ascertained all this, Citragupta reports it; thereafter Citragupta tells him [Yama] all the person's deeds.” (2.18.2)
कृतकर्म लिखत्यसौ/ krtakarma likhaty asau/ “He records the deeds that have been done” (2.33.29).
And because Chitragupta records and tells Yama of one’s deeds, this is a day of some reckoning. The Priest tells us about a book which gives the story of Chitragupta, which is bought and handed out on this day. When we go to find it at Giri trading, it’s sold out (we are so amused, as is the priest when we report that to him later).
The Chitragupta Puja here consists of the following:
3:30pm, a “vilakku puja” in which kutthu vilakku (velli deepa sthambam) and a drawing on cloth with a standing figure stands in for Chitra Gupta. The reading of the story of Chitragupta happens directly after and takes an hour. Notebooks, pens are distributed, along with a book of the stories which used to be written by hand and given out in days before xerox.
5pm: the customary Pournami Satyanarayana puja
By 8pm: Chandra bhojanam. All the food is made in the temple madapalli, and they used to eat on the terrace of the neighbouring kalyana mandapam when there was no roof there. (Arun’s mother recalls going out and sitting on riverbeds on Chithirai pournami at night, during her growing up days in Tirunelveli).
This mixing of the different meanings of Chithirai pournami is part of what is confusing. The Chitragupta story and its connection to the pournami appear to be strictly speaking homophonic–the names sound alike, but there is properly no connection, except now of course in practice and popular explanation.
In any case there are many pujas we miss on this day, including a Sridevi Ezhai Mariamman theruvu koyil on Perumal Koyil street, doing a 108 Shankhabhishekam (morning), and the vilakku puja at the Lada Amman Koyil/Chetty Koyil.