Narada Purana (Jyotisha Selections) · by Vyasa
Chapter 7 · Samskara Muhurtas
संस्कारमुहूर्ताः
नवमे सप्तमे वापि पञ्चमे दिवसेऽपि वा ।तृतीये बीजनक्षत्रे शुभवारे शुभोदयें ॥ ३४४ ॥
In the full Narada Purana (Purva Bhaga) this is chapter 56, verse 344.
Do it on the 9th, 7th, 5th or 3rd day before the rite. Use a star good for sowing seed, a good weekday and a good lagna (rising sign).
Simple meaning: Choose the 9th, 7th, 5th or 3rd day before the rite, a good star, weekday and lagna.
आधानादष्टमे वर्षे जन्मतो वाग्रजन्मनाम् ।
The sacred thread is tied in the 8th year for Brahmins, the 11th for kings and the 12th for Vaishyas.
जन्मतः पंचमे वर्षे वेदशास्त्रविशारदः ।
The sacred thread in the 5th year can make a boy learned and prosperous.
बालस्य बलहीनोऽपि सितो जीवः शुभप्रदः ।
Venus and Jupiter help a boy even when weak. Keep to the years given for the rite.