In Shodash Sanskaras "The Sacred Thread Ceremony" is an most important Sanskara. In Hinduism, the "sacred thread" (Sanskrit: yajñopavitam or upavita) is a thin, consecrated cord, composed of distinct cotton strands, worn to symbolize the permission given to the wearer to perform sandhyavandanam and recite the Gayatri Mantra.
Without wearing Yagyopaveet one does not attain success in any type of ritualistic activities like Puja-Paath, Yagya-Anushthaan, Siddhi or Sadhana etc. Therefore it is essential to perform sacred thread ceremony. This Sanskaar is applicable to males only.
Yagyopaveet of Brahmin & Kshatriya boy should be done at the age of 5 or 8 and that of Vaishya at 6 or 11. Getting it done after the above given age gives average results.
It should be done in Kshiprasangyak, Dhruvasangyak, Ashlesha, Charsangyak, Moolmridusangyak, all 3 Poorva & Ardra Nakshatras, on Sunday, Wednesday, Thursday, Friday, Monday and Dwitiya, Tritiya, Panchami, Ekadashi & Dwadashi Tithi in the morning before noon.
The main ceremony should be performed in auspicioius Muhurat and the other activities should be done as per family/social tradition.