The affix yat comes in the chandas (Vedas) with the force of matup after a word in the Nominative 1st-Case in construction, the word so formed meaning 'a month' or 'a body'.,
This debars the affix मतुप् and those having the sense of मतुप् ॥ Thus नभांसि विद्यन्ते यस्मिन् मासे = नभस्यः 'the month of clouds' i. e. June - July. So also सहस्य, तपस्यः मधव्यः, being the names of the months Pausha (Nov-Dec), Phalguna (Feb-March) and Vaisakha (April-May). The word मधव्यः is thus formed:- मधु + यत् = मधा + यत् 6.4.146 = मधव्यः 6.1.79 'the month of honey'. Similarly ओजस्या तनूः 'the bodies full of vigour'.
Why do we say \in denoting a month or a body\? Observe मधुमता पात्रेण चरति, here मतुप् is used.
Vart:- The affix यत् is optionally employed when the sense is 'immediately after'. As मध्वस्मिन्नस्ति मध्वस्मिनन्तरमिति वा = माधव्यः or माधवो मासः ॥
Vart:- Sometimes the affix is elided, and sometimes the letters अ, इ and र serve the same purpose as this affix. Thus तपः or तपस्यः, नभस् or नभस्यः, सहस् or सहस्यः, all in the neuter gender in the Chhandas. Sometimes अ serves the same purpose, e. g. इषो मासः, ऊर्जोमासः ॥ Sometimes इ, as शुचिर्मासः. Sometimes र e. g. शुक्रो मासः ॥,