Praise be to Allah Who has created time and has made some times better than others, some months and days and nights better than others, when rewards are multiplied many times, as a mercy towards His slaves. This encourages them to do more righteous deeds and makes them more eager to worship Him.
This season of worship brings many benefits, such as the opportunity to correct one’s faults and make up for any shortcomings or anything that one might have missed. Each of these special occasions involves some kind of worship through which people may draw closer to Allah, and some kind of blessing through which Allah bestows His favour and mercy upon whomsoever He, Almighty, Wills.
The Muslim must understand the value of his life, increase his worship of Allah and persist in doing good deeds until the moment of death. Allah Almighty says (what means): “And worship your Lord until there comes to you the certainty (death).” [Qur’an 15:99] The commentators said: “’The ‘certainty’ means death.”
Among the special seasons of worship are the first 10 days of Dhul-Hijjah, which Allah has preferred over all the other days of the year. Ibn ‘Abbaas, may Allah be pleased with him, reported that the Prophet, sallallaahu ‘alaihi wa sallam, said: “There are no days in which righteous deeds are more beloved to Allah than these 10 days.” The people (the Companions, may Allah be pleased with them) asked: “O Messenger of Allah, not even Jihad in the path of Allah?” He, sallallaahu ‘alaihi wa sallam, said: “Not even Jihad in the path of Allah, except in the case of a man who went out to fight giving himself and his wealth up for the cause, and came back with nothing (lost both his property and life).” [Al-Bukhari]
This text and others indicate that these 10 days are better than all the other days of the year, with no exceptions, not even the last 10 days of Ramadan. But the last 10 nights of Ramadan are better, because they include the Night of Al-Qadr (Decree), which is better than a thousand months. Thus the various reports may be reconciled. [See Tafseer Ibn Katheer, 5/412]

The virtues of these 10 days:
The virtues are based on many things:
1.    Allah Swears an oath by 10 days of Dhul-Hijjah, and swearing an oath by something is indicative of its importance and great benefit. Allah says (what means): “By the dawn and by the 10 nights (i.e. the first 10 days of the month of Dhul-Hijjah)…” [Qur’an 89: 1-2]
Ibn ‘Abbaas, Ibn Az-Zubayr, Mujaahid and others of the earlier and later generations, may Allah have mercy upon them, said that this refers to the first 10 days of Dhul-Hijjah. Ibn Katheer, may Allah have mercy upon him, said: “This is the correct opinion.” [Tafseer Ibn Katheer, 8/413]
2.    The Prophet, sallallaahu ‘alaihi wa sallam, encouraged people to do righteous deeds because of the virtue of this season for people throughout the world, and also because of the virtue of the place - for the pilgrims to the Sacred House of Allah.
3.    The Prophet, sallallaahu ‘alaihi wa sallam, commanded us to recite a lot of Tasbeeh (“SubhaanAllah”), Tahmeed (“Al-hamdu Lillaah”) and Takbeer (“Allahu Akbar”) during this time. ‘Abdullaah Ibn ‘Umar, may Allah be pleased with him, reported that the Prophet, sallallaahu ‘alaihi wa sallam, said: “There are no days greater in the sight of Allah and in which righteous deeds are more beloved to Him than these 10 days, so during this time recite a great deal of Tahleel (“Laa ilaaha illAllah”), Takbeer and Tahmeed.” [Ahmad]
4.    These 10 days include the Day of ‘Arafah, on which Allah perfected His Religion. Fasting on this day will expiate for the sins of two years. These days also include the Day of Sacrifice, the greatest day of the entire year and the greatest day of Haj, which combines acts of worship in a way unlike any other day.

Shall you offer a sacrifice?
The Sunnah indicates that the one who wants to offer a sacrifice must stop cutting his hair and nails and removing anything from his skin, from the beginning of the 10 days until after he has offered his sacrifice, because the Prophet, sallallaahu ‘alaihi wa sallam, said: “When you see the new moon of Dhul-Hijjah, if any one of you wants to offer a sacrifice, then he should stop cutting his hair and nails until he has offered his sacrifice.” According to another report he, sallallaahu ‘alaihi wa sallam, said: “He should not remove (literally, touch) anything from his hair or skin.” [Muslim]
The Prophet’s, sallallaahu ‘alaihi wa sallam, instruction here makes one thing obligatory and his prohibition makes another Haraam (prohibited), according to the soundest opinion, because these commands and prohibitions are unconditional and unavoidable.
However, if a person does any of these things deliberately, he must seek Allah’s forgiveness but is not required to offer (an extra) sacrifice in expiation; his sacrifice will be acceptable. Whoever needs to remove some hair, nails, etc. because it is harming him, such as having a broken nail or a wound in a site where there is hair, can do so, and there is nothing wrong with that. The state of Ihraam is so important that it is permitted to cut one’s hair if leaving it will cause harm. There is nothing wrong with men or women washing their heads during the first 10 days of Dhul-Hijjah, because the Prophet, sallallaahu ‘alaihi wa sallam, only forbade cutting the hair, not washing it.
The wisdom behind this prohibition of the one who wants to offer a sacrifice from cutting his hair, etc., is so that he may resemble those in Ihraam in some aspects of the rituals performed, and so that he may draw closer to Allah by offering the sacrifice. So he leaves his hair and nails alone until the time when he has offered his sacrifice, in the hope that Allah will save him in his entirety from the Fire. And Allah knows best.
If a person has cut his hair or nails during the first ten days of Dhul-Hijjah because he was not planning to offer a sacrifice, then he decides later, during the 10 days, to offer a sacrifice, then he must refrain from cutting his hair and nails from the moment he makes this decision.
Some women may delegate their brothers or sons to make the sacrifice on their behalf, then cut their hair during these 10 days. This is not correct, because the ruling applies to the one who is offering the sacrifice, whether or not he (or she) delegates someone else to carry out the actual deed. The prohibition does not apply to the person delegated, only to the person who is making the sacrifice, as is indicated in the narration. The person who is sacrificing on behalf of someone else, for whatever reason, does not have to adhere to this prohibition.
This prohibition appears to apply only to the one who is offering the sacrifice, not to his wife and children, unless any of them is offering a sacrifice in his or her own right, because the Prophet, sallallaahu ‘alaihi wa sallam, used to sacrifice “on behalf of the family of Muhammad,” but there are no reports that say he, sallallaahu ‘alaihi wa sallam, forbade them to cut their hair or nails at that time.
If a person was planning to offer a sacrifice, then he decides to go and perform Haj, he should not cut his hair or nails if he wants to enter ihraam, because the Sunnah is only to cut hair and nails when necessary. But if he is performing Tamattu’ [whereby he performs ‘Umrah, comes out of ihraam and enters ihraam anew for Haj], he should trim his hair at the end of his ‘Umrah because this is part of the ritual.