Surya Siddhanta
कार्तिक्यादिषु सम्योगे कृत्तिकादि द्वयम् द्वयम्। अन्त्योपान्त्यौ पञ्चमश् च त्रिधा मासत्रयम् स्मृतम्॥
From the full moon of the month of Kärttka* , a month is named from the conjunction of two constellations. But for the months of Açvina**, Bhädra*** and Phälguna**** the months are named after the conjunction of three constellations. [* The 7th month of the year. **The 6th month of the year. ***The 5th month of the year. ****The 11th month of the year.]
english translation