Narada Purana (Jyotisha Selections) · by Vyasa
Chapter 7 · Samskara Muhurtas
संस्कारमुहूर्ताः
दृश्यमानगुरौ शुक्रे शाखेशे चोत्तरायणे ।वेदानामधिपा जीवशुक्रभौमबुधाः क्रमात् ॥ ३५१ ॥
In the full Narada Purana (Purva Bhaga) this is chapter 56, verse 351.
Do it when Jupiter, Venus and the shakhesha (lord of the person's Veda branch) are visible. Do it in the Sun's northward course. Jupiter, Venus, Mars and Mercury rule the Vedas, in order.
Simple meaning: Do the thread rite when Jupiter, Venus and the lord of one's Veda are visible, in the Sun's northward course.
शरद्ग्रीष्मवसंतेषु व्युत्क्रमात्तु द्विजन्मनाम् ।
Autumn, summer and spring, in reverse order for the three twice-born groups, are best. Five months from Magha are ordinary.
स्वकुलाचारधर्मज्ञो माघमासे तु फाल्गुने ।
A thread rite in Magha brings family knowledge, in Phalguna knowledge of rules, in Chaitra wealth and learning.
वैशाषे धनवान्वेदशास्त्रविद्याविशारदः ।
A thread rite in Vaishakha brings wealth and learning. In Jyeshtha it brings strength and skill in rites.