A nail is a horn-like keratinous envelope covering the tips of the fingers and toes in most primates. Nails evolved from claws found in other animals. Fingernails and toenails are made of a tough protective protein called alpha-keratin which is found in the hooves, hair, claws and horns of vertebrates.
If it is 'dirt', and it is accessible, and would prevent water from getting to your out layer of skin, then it is wajib to remove, for the purpose of wudhu and ghusl.
If the dirt is not accessible, as in it is a stain under your nail that you cannot remove, or that the dirt is not an obstacle and will not prevent water from getting to your skin, then you do not have to remove it, and your wudhu/ghusl will be valid.
And Allah knows best.
Clipping of our nails is considered an important hygienic practice in Islam. In our sunnah it speaks about many physical and spiritual benefits of clipping nails.
Among the etiquette of cutting nails is that one should cut their nails on a Friday, and start from the little (pinky) finger of their left hand, and end with the little finger of their right hand.
Other days that have been mentioned that would be good to cut nails is Tuesday and Thursday.
It is also mentioned that one should bury their nail clippings, or throw it into nature.
It is makruh to rip off your nails with biting them off.
As for cutting nails during the night, there is no evidence for it being bad or makruh.
In regards to hair hygene, Islam emphasises on this a lot as well. Especially having clean hair, and brushing/combing of hair. As for cutting of hair in the night time or day time, there is also no evidence that mentions it is bad to cut at night.
And Allah knows best.
If the long nails stop water from reaching to any part of the hand which must be washed in Wudhu, it will then cause the Wudhu to be invalid. But if you ae sure that water is reaching to every part of your hand which must be washed in Wudhu, then you have no problem.
It is better to avoid long nails as it can cause some times preventing water from reaching and if such thing happens, the Wudhu will be invalid.
Regarding the Islamic etiquette of cutting nails, here are some points to consider:
1. It is recommended to cut nails on Fridays, and it is also the best day to do so.
2. Other days that have been mentioned as recommended days for cutting nails are Tuesday and Thursday.
3. If you do cut your nails on a Wednesday, start with your right pinky nail, and end with your left pinky nail.
4. It is very detested to chew on nails, or cut nails with teeth.
5. When you cut your nails, say:
بِسْمِ اللّهِ وَبِاللّهِ وَعَلى سُنَّةِ مُحَمَّدٍ وَآلِ مُحَمَّدٍ
6. You should throw your nails outside, or bury them outside.
7. It is recommended for women to grow their nails, as it is a form of women's zeenah.
8. As for some narrations that speak about it being bad cutting on Wednesday, I was not about to trace the authenticity of this statement.
And Allah knows best