Shiva Purana
अर्कवारे तथार्द्रायां पञ्चदश्यां च पक्षयोः ॥ अष्टम्यां च चतुर्दश्यां शक्तस्तूपवसेदपि ॥ ६५ ॥
If he is strong enough he shall observe fast on Sundays on Ardrā star days, on full moon and new moon days and on the eighth and fourteenth days.
english translation
arkavAre tathArdrAyAM paJcadazyAM ca pakSayoH ॥ aSTamyAM ca caturdazyAM zaktastUpavasedapi ॥ 65 ॥
hk transliteration by Sanscript