As Salaamu Alaikum!
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 ten days until after he has offered his sacrifice, because the Prophet (Sallallahu Alaihi Wassallam) said: "When you see the new moon of Dhu’l-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 said: "He should not remove (literally, touch) anything from his hair or skin." (reported by Muslim with four isnaads, 13/146)
There is nothing wrong with men or women washing their heads during the first ten days of Dhu’l-Hijjah, because the Prophet(Sallallahu Alaihi Wassallam) only forbade cutting the hair, not washing it.
The wisdom behind this prohibition, probably, is that 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 Subhanahu wa Ta'ala 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 Subhanahu wa Ta'ala 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 Dhu’l-Hijjah because he was not planning to offer a sacrifice, then he decides later, during the ten days, to offer a sacrifice, then he must refrain from cutting his hair and nails from the moment he makes this decision. And Allah knows best.
The things that are described above as being prohibited for the person who is planning to offer a sacrifice are reported in the hadeeth quoted above; the person is not forbidden to wear perfume, have marital relations, wear sewn garments, etc.
The Muslim should make sure that he does not miss any of these important occasions, because time is passing quickly. Let him prepare himself by doing good deeds which will bring him reward when he is most in need of it, for no matter how much reward he earns, he will find it is less than he needs; the time of departure is at hand, the journey is frightening, delusions are widespread, and the road is long, but Allah Subhanahu wa Ta'ala is ever watchful, and to Him will we return and render account. As the Qur’aan says (interpretation of the meaning):
"So whosoever does good equal to the weight of an atom, shall see it,
And whosoever does evil equal to the weight of an atom, shall see it." [al-Zalzalah 99:7-8]
There is much to be gained, so make the most of the opportunity afforded by these invaluable and irreplaceable ten days. Hasten to do good works, before death strikes, before one can regret one’s negligence and failure to act, before one is asked to return to a place where no prayers will be answered, before death intervenes between the hopeful one and the things he hopes for, before you are trapped with your deeds in the grave.
O you whose hard heart is as dark as the night, is it not time that your heart was filled with light and became soft? Expose yourself to the gentle breeze of your Lord’s mercy during these ten days, for Allah Subhanahu wa Ta'ala will cause this breeze to touch whomever He wills, and whoever is touched by it will be happy on the Day of Judgement. May Allah bless our Prophet Muhammad (Sallallahu Alaihi Wassallam) and all his Family and Companions. Aameen!
With Best Wishes
