Shodashi Vigyan:

The sixteen letter mantra of Shodashi is called Shodashi Vidya itself. The sixteen letters are the sixteen nityas(eternals).

The last letter of Luxmi beej is called Nitya. It is PARA kala and because of it whole vidya is called Sri Vidya. This is the sixteenth kala which established as Chandrakala in our Sahasrar and this only is reflected on the sixteen petals of Vishudhi Chakra. Light of the first kala starts from East and circumambulating locates itself on the  16^th petal in the NE(Ishan)-East direction. Taking eight directions and inbetween direction we come to a figure of sixteen, which is the no of petals of Vishudh chakra. Thus all the kalas move and establish themselves on the respective petal of the Vishudhi chakra. All these kalas come under spell of the 16^th kala which is totally free. Various dates of Krishan paksh and shukla paksh and Poornima or Amavasya are all Chandra kalas. They arise in Shukla paksh in conjunction with Sun and end in Sun in Krishan paksh.

Pratham kala arises on Pratipada of Shukla paksh and sets down on Pratipada of Krishan paksh.

Dwitya kala moves from Dwitya of Shukla paksh to Dwitya of Krishan paksh. Hence all the other kalas move on their respective tithis(dates).

Pornima kala of Purnima sets down in Amavasya.

On Amavasya when Poornima kala sets then what ever Chandra kala is left is called SHODASHI nitya kala. This energy of the Chandra shines with Sun in the various increasing decreasing kalas.

Chit Shakti’s fifteen kalas are represented by fifteen letters of the Panchdashi mantra. The sixteen kala of pure chit shakti Mahatripursundari established in Nirvikalp Samadhi. Reflection of moon is always same, bereft of increase or decrease, hence every kala should be understood as Shodashi kala only. Poojan and dhyan of each is done on that very tithi along with Shaodashi kala.  

When Kundlini rises up, it pierces the Chandra mandal and through that aperture nectar descends, inundates Ajna/Agya Chakra. Because of it all the sixteen kalas of Chandra start shining there for ever and hence they are called Nityas. Then they descend down to Vishudhi chakra and illuminate the sixteen petals. Chandra mandal located in the middle of Sahasrar is called Vaindavsthan.This is the place of Anandmayi kala of Shudh Chit Shakti. This itself is called SHREE or MAHATRIPURSUNDARI.

Tripura devi has two sides, one is called Samvit and other is called Vimarsh. Lauhitya Varna of Srividya is called its PRAN.

As per Acharya Abhinavgupt, nectar continuously flows from Chandra. It is of two types,

1 In the form of evident kalas of jyotish chakra Chandra. Up to Chaturdashi before Amavasya. All the devtas drink it. Karan devis drink it in their breath.

2 In the form of purest Ap/ अप (water) tatva. Devtas can drink only the fifteen kalas which keeps on decreasing and ends by Amavasya (night of no moon). Devtas, Pitr and Humans drink their nectar, which decreasing dissolves in to the Ama(अमा). That Ama kala which was left in the last is Sixteenth or Shodashi kala and is protected in the middle of Guha. The fifteen kalas stay in it and arise out of it during Shukla paksh. Because of this AVAS of AMA kala it is called AMAVASYA/ अमावस्य. Ama kala/ अमा कला is subtle water element and hence Sun also can not dry it up or drink it. Between Amavasya/ अमावस्या and Pratipada/ प्रतिपदा there is a place which is called Bindu/ बिन्दु where imagination of tithi is not even possible. During Amavasya, moon completely dissolves in Sun, gets heated up and starts flowing nectar. Shukla paksh/ शुक्ल पक्ष tithi are in fact the presiding deities of Sri Chakra, while the ama kala is Sadashiv/ सदाशिव, which itself is the presiding deity of all the fifteen kalas or devis. First five kalas of Panchdashi represent AGNI, next five represent SUN/SURYA while the other five are called SOM(सोम)/CHANDRA/MOON(चन्द्र). SHREE/ श्री is the sixteenth kala and is beyond the fifteen and is also called Madhukari/ मधुकरि.

As per Taitirya Upnishad Bra.3.10.10, ‘Iyam Vaav Sargha’.

Sargha = Honey  Bee.

Since a bee develops honey at night hence Yogis also raise their kundlini not during day but at night. Thus Shree upasak also perform their anushthaans during Shukla paksh only ( i.e during the nights of waxing moon). Since nectar descends during the day, hence meditation of kundalini during the day is not beneficial, it is done only during night.

Such is the explanation given in Sri Vidya Sadhana by Shyamakant Dwivedi published in Chaukhamba Surbharti Granth Mala 391.