The affix aṇ already ordained -- prāgdīvyato'ṇ [[4.1.83]], comes after a word in the Instrumental -3rd case in construction which is the name of a lunar mansion, to signify a time connected with the asterism.,
A time is said to be connected with an asterism when the moon is in conjunction with any one of such asterism. Thus पुष्य, तिष्य, मघा &c are lunar asterisms, when the moon is in any one of these asterisms, then the affix is added to the name of such asterism in order to denote time of such conjunction. Thus पुष्य + अण् = पौषः 6.4.149 e.g. पौषिरात्रिः \the night in which moon is in Pushya\. So also माघी रात्रिः; पौषमह 'the month in which the moon is full in Pushya i. e. December', so also माघमह 'the month Magha' &c.
Why do we say नक्षत्रेण 'connected with the asterism'? Observe चन्द्रमसा युक्ता रात्रिः ॥ Why do we say काल 'time'? Observe पुष्येण युक्तचन्द्रमाः ॥,