Short answer
Upanayanam (the sacred-thread ceremony) is traditionally timed by the family priest, who weighs the child's age, the season and a muhurta for your town. Many families prefer the Uttarayana months and avoid the child's Chandrashtama days and Rahu Kalam. For this or any family ceremony, use Find Good Times to shortlist dates before you see the priest.
How ChiChakra reads this
ChiChakra calculates the day's Panchangam, Rahu Kalam and Abhijit Muhurta for any city, and Chandrashtama from a Moon sign. Ceremony-specific rules, such as the right age or month, differ by family and community, so the tool narrows options and the priest decides.
Organising any family ceremony
Name the ceremony, the town and the dates your household can genuinely manage.
Check them in Find Good Times, drop any on the main participant's Chandrashtama days, and plan the key ritual outside Rahu Kalam.
Take two or three options to the priest before paying any deposits, and keep one fallback date in case plans change.
Good to know
For Upanayanam especially, regional rules vary widely, and your priest's calendar is the final word.