Narada Purana (Jyotisha Selections) · by Vyasa
Chapter 6 · Calendar, Muhurta Elements
कालमानं मुहूर्ताङ्गानि च
कृष्णा पंचदशी माघे द्वापरादिमुदीरिता ।कल्पादिः स्यात्कृष्णपक्षे नभस्यस्य त्रयोदशी ॥ १४८ ॥
In the full Narada Purana (Purva Bhaga) this is chapter 56, verse 148.
The dark 15th in Magha is called the first day of the Dvapara age. The start of a kalpa (a great age of the world) is the dark 13th of Nabhasya.
Simple meaning: The Dvapara age begins on the dark new moon of Magha, and a kalpa on the dark 13th of Nabhasya.
द्वादश्यूर्जे शुक्लपक्षे नवम्यच्छेश्वयुज्यपि ।
More holy days: bright 12th in Urja and bright 9th in Ashvina. The bright 3rd of Chaitra and Bhadrapada is called Shukla.
एकादशी सिता पौषे ह्याषाढेर्देशमीसिता ।
More holy days: the bright 11th of Pausha, the 10th of Ashadha, the 7th of Magha and the dark 8th of Nabhasya.
श्रावणे मास्यमावास्या फाल्गुने मासि पौर्णिमा ।
More holy days: new moon of Shravana, and full moons of Phalguna, Ashadha, Kartika, Jyeshtha and Chaitra.