Photo Source - By RKK Energia (nssdc.gsfc.nasa.gov) [Public domain], via Wikimedia Commons
If you are a devotee of God & are connected with Him by Yoga (Bhaktiyoga, Gyanyoga, Dhyanyoga etc.) then it is advisable not to cut hair, nail etc. on No Moon day (Amawasya)
All presence (Vyapakta) of demi-god whom we worship is strong during auspicious days, also Amavasya.
Cutting hair is not advisable as
1) Union of God-devotee or
2) Soul Union or
3) Psychic field union develops mismatch.
See Prahlad Jani, refer Wikipedia.
He has not eaten food or dranked water since 70+ years. His appearance is that of a goddess. What programs him to appear like her?
Also cutting nails, hair activities can create disturbance in minds of demi-gods as we remain connected strongly during these days. So if we give disturbance, the resultant effect might be a reverse disturbance because we earn a sin. Subconscious is reflecting the SAT-Shakti. Eg. Lord Shiva says "Bring that baby whose mother is having back towards it."(For placing head for Lord Ganesh) Consciously mother loves the baby but the incident indicates the subconscious. Subconscious is more true. Hence if you cut nails or hair, your subconscious is revealed that you intend to disturb the God. This taints your Sata/Sat. But if you could control then this means you are an awakened, strong, active soul. You deserve the bliss of demi-gods. Your Sat is pure white, means you are Pure.
Refer below link for more knowledge
What is Vyapakta or ALL-PRESENCE?
Refer the link Hindu Milk Miracle.
This INCIDENT shows the all presence or Vyapakta of Deity Ganesh. That way when we are in touch with any deity, their vyapakta or all presence covers us. Hence it is necessary to follow rules of peace & purity. Due to this some people advise that it is better not to cut hairs, nails etc. as they(Deities whom we worship) can get the feel, which we are getting. It is our duty to give feels of worship, hence we put flowers, milk etc. on their statues. Ill or sinful thoughts during worship can generate a backfire resulting to shocking incidents in our life. Worshipping Lord Vishnu is advisable as He is a very tolerant God and usually if sinful thoughts grabs mind there is probably no backfire. As Gods are Vyapak or all present, the backfire generated in their minds towards ill thoughts have a capability of converging event matrix of nature, means their vibes can govern & affect the external nature in which we live.
GIVE EVERY MAN THY EAR
BUT RESERVE THY JUDGEMENT
This is the sign of a free thinker. Rest are Brain washed victims of modern universities.
So this is a Superstition or Spiritual Science, let your reasoning decide it.