Significance of the 1st 10 days of Zul Hijjah

عَنْ ابْنِ عَبَّاسٍ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: "مَا مِنْ أَيَّامٍ العَمَلُ الصَّالِحُ فِيهِنَّ أَحَبُّ إِلَى اللَّهِ مِنْ هَذِهِ الأَيَّامِ العَشْرِ"، فَقَالُوا: يَا رَسُولَ اللَّهِ، وَلَا الجِهَادُ فِي سَبِيلِ اللَّهِ؟ فَقَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: "وَلَا الجِهَادُ فِي سَبِيلِ اللَّهِ، إِلَّا رَجُلٌ خَرَجَ بِنَفْسِهِ وَمَالِهِ فَلَمْ يَرْجِعْ مِنْ ذَلِكَ بِشَيْءٍ"

There is no virtuous deed carried out in any day of the year that can be more virtuous than (that virtuous deed carried out in) these ten days (of Zul Hijjah)”. The Sahabaah (radiyAllahu anhum) asked, “Not even jihaad?” Rasulullaah (sallAllahu alayhi wasallam) replied, “Not even jihaad, except for the one who endangers his life and wealth (in jihaad) and does not return with anything.” (Bukhaari) 
The significance of these days and nights can be gauged from the fact that Allaah Ta’ala Himself took an oath on them: “I swear by the dawn, and by the Ten Nights.” (Surah Fajr)
Many Sahaabah and others have explained that these ten nights refer to the first ten nights of Zul Hijjah. (Tafseer Ibn Katheer) Allaah Ta’ala takes an oath to show the importance of the thing upon which He is taking an oath. Thus the importance of these ten nights can be understood from this. Rasulullaah (sallAllahu alayhi wasallam) has also explained the virtue of these ten days and has encouraged us to take full advantage of them

The great commentator of Bukhaari, Haafiz Ibn Hajar Asqalaani (rahimahuAllah) explains: “The reason for the excellence of the first ten days of Zul Hijjah is that the main kinds of ibaadat, viz. salaah, fasting, charity and hajj, are all carried out in them, and not in the other days of the year.” (Fathul Baari) It is for this reason that many scholars are of the opinion that these days and nights are even greater than the last ten days and nights of Ramadhaan. However, others opine that the days of Zul Hijjah are greater and the nights of Ramadhaan are greater. (Tafseer Ibn Katheer)

ACTIONS TO BE OBSERVED ON THESE DAYS

1. Fasting: “The fasts of each of these days are equal to the fast of an entire year.” (Tirmidhi)
Fasting on the day of Arafah holds a special virtue - forgiveness of one past year’s sins and one future year’s sins. (Muslim)

2. Worship at night: “The worship of each of these nights is equal to the worship of Laylatul Qadr.” (Tirmidhi)

3. “Increasingly recite tasbeeh (subhaanallah), tahleel (laa ilaaha illallah), tahmeed (alhamdulillah), takbeer (Allahu akbar) during the first ten days of Zul Hijjah.” (Majmauz Zawaaid)
Abdullah ibn Umar and Abu Hurairah (radiyAllahu anhuma) would go to the market place during these ten days, reciting takbeer, and the people would follow them in reciting. Imaam Tahaawi (rahimahuAllah) says, “Our mashaayikh were of the opinion that takbeer be recited during the ten days.” (Fathul Baari) Mufti Muhammad Taqi Usmaani (hafizahullah) explains, “This takbeer is apart from the takbeer tashreeq which is waajib after the fardh salaah. Rather, it is mustahab that during the first ten days of Zul Hajjah, takbeer be recited excessively, while walking, standing, sitting, at home and in the market place. It seems as though, Allaah Ta’ala desires that His greatness be extoled during these days. It is permissible to recite this takbeer softly or aloud. In our era this has been abandoned, whereas this recitation has been reported from many Sahaabah (radiyAllahu anhum). Hence, we should practice on this. (In‘aamul Baari)

4. Hair and Nails: It is mustahab for those intending qurbaani not to clip their nails nor trim their hair from the beginning of the month of Zul Hijjah till their qurbaani animal is slaughtered. Rasulullaah (sallAllahu alayhi wasallam) said, “Whoever intends making qurbaani, then he should refrain from cutting his hair and clipping his nails from the time the crescent (of Zul Hijjah) is sighted till he completes his qurbaani.” (Muslim)

5. Qurbaani: “There is no actions dearer and more beloved to Allaah Ta’ala during the days of qurbaani than the spilling of blood (i.e. the sacrificing of animals). Verily it (the sacrificed animal) will come on the Day of Qiyaamah with its horns, hair and hooves (to be weighed on the scale of good deeds); and indeed it is accepted by Allaah before its blood reaches the ground, therefore sacrifice with a happy heart.” (Tirmidhi)