सा ऽस्मिन् पौर्णमासी इति सञ्ज्ञायाम्

The word स means 'that'. It is in the nominative case, therefore by 4.1.82 it means that \after a word in the first case in construction\. अस्मिन् 'in it' i.e. in the sense of a seventh case. पौर्णमासीति \if the word in the first case in construction is the name of a full-moon night\. The word इति after पौर्णमासी is used for the sake of clearness. The word संज्ञायाम् does not occur in the original sutras of Panini; but is added by a Vartika; and means 'if the word so formed is an appellative', i. e. if it is the name of a month, a half-month and a year. The whole sutra therefore literally translated means \the affix अण् already ordained (IV. I. 83) comes in the sense of a 7th case affix, after a word in the Ist case in construction, if that word is the name of a full-moon-night, provided that the whole word so formed is an appellation of a month, a half-month or a year\.

Thus पौषी पौर्णमासी अस्मिन् = पौषो मासः 'the month Paus i. e. that month in which the moon is full in the asterism Pushya'. Similarly पौषऽर्द्धमासः and पौषः संवत्सरः ॥ But not so in the following पौषी पौर्णमासी अस्मिन् दशरात्रे, because dasaratra is not a month or a fortnight or a year.

The words इति and संज्ञायाम् have the same force here, namely, they make the word an appellative. The word संज्ञायाम् added to the sutra by the Vartika is a redundancy.

The word पौर्णमासी is formed by adding अण् to पूर्णमास ॥ Or it may be evolved in this way:-पुर्णो माः = पूर्णमाः, पूर्णमास इयम् पौर्णमासी ॥ The word माः means \moon\.,