The affix aṇ comes in the sense of 'this is the principal food on this particular occasion' after the word kulmāṣa when the whole word is a Name.,
The ञ् of अञ् is for the sake of causing Vriddhi and accent. Thus कुल्माषाः प्रायेणान्नमस्यां = कौल्माषी पौर्णमासी ॥,