What are the Merits of the ten days of Dhul-Hijjah?
Question

Are the first ten days of Dhul-Hijjah more special than other days? What are the righteous deeds that it is recommended to do a lot of during these ten days?
Praise be to Allah.Merits of the ten days of Dhul-Hijjah
Among the great seasons of worship are the first ten days of Dhul-Hijjah, which Allah has favoured over other days. Ibn ‘Abbas (may Allah be pleased with him) narrated that the Prophet (peace and blessings of Allah be upon him) said: “There are no days on which righteous deeds are more beloved to Allah than these ten days.” They said: “Not even jihad for the sake of Allah?” He said: “Not even jihad for the sake of Allah, unless a man goes out himself for jihad taking his wealth with him and does not come back with anything.” (Narrated by Al-Bukhari, 2/457)
Anas ibn Malik (may Allah be pleased with him) narrated that the Prophet (peace and blessings of Allah be upon him) said: “There is no deed that is better before Allah or more greatly rewarded than a good deed done in the (first) ten days of Al-Adha.” It was said: “Not even jihad for the sake of Allah?” He said: “Not even jihad for the sake of Allah, unless a man goes out himself for jihad taking his wealth with him and does not come back with anything.” (Narrated by Al-Darimi, 1/357; its isnad is hasan (good) as stated in Al-Irwa’, 3/398) 
These texts and others indicate that these ten days are better than all other days of the year, with no exception, even the last ten days of Ramadan. But the last ten nights of Ramadan are better than the first ten nights of Dhul-Hijjah, because they include Laylat al-Qadr, which is better than a thousand months. (See Tafsir Ibn Kathir, 5/412.)
Recommended deeds during Dhul-Hijjah
So, the Muslim should start these days with sincere repentance to Allah, then he should do a lot of righteous deeds in general, then he should pay attention to doing the following deeds: 
1 – Fasting 
It is Sunnah for the Muslim to fast on the first nine days of Dhul-Hijjah . This is because the Prophet (peace and blessings of Allah be upon him) encouraged us to do righteous deeds in these days. Fasting is one of the best of deeds, which Allah has chosen for Himself as it says in the hadith qudsi: “Allah says: ‘All the deeds of the son of Adam are for him, except fasting, which is for Me and I shall reward for it.” (Narrated by Al-Bukhari, 1805)
The Prophet (peace and blessings of Allah be upon him) used to fast on the first nine days of Dhul-Hijjah. It was narrated from Hunaydah ibn Khalid from his wife, that one of the wives of the Prophet (peace and blessings of Allah be upon him) said: The Prophet (peace and blessings of Allah be upon him) used to fast on the first nine days of Dhul-Hijjah and the day of ‘Ashura’, and three days each month, the first Monday of the month and two Thursdays. (Narrated by Al-Nasa’i, 4/205 and by Abu Dawud; classed as saheeh by al-Albaani in Sahih Abi Dawud, 2/462)
2 – Reciting tahmid, tahlil and takbir a great deal 
It is Sunnah to recite takbir  (saying Allahu akbar (Allah is most great)), tahmid (saying al-hamdu Lillah (praise be to Allah)), tahlil (saying La ilaha ill-Allah (There is no god but Allah)) and tasbih (saying Subhan-Allah (Glory be to Allah)), during the first ten days of Dhul-Hijjah, saying these words out loud in the mosques, homes, streets and every place in which it is permissible to remember Allah, to perform this act of worship openly and proclaim the greatness of Allah. 
Men should recite out loud and women should recite quietly. 
Allah says (interpretation of the meaning): 
“That they may witness things that are of benefit to them (i.e. reward of Hajj in the Hereafter, and also some worldly gain from trade), and mention the Name of Allah on appointed days, over the beast of cattle that He has provided for them (for sacrifice).” [Al-Hajj 22:28] 
The majority are of the view that the “appointed days” are the first ten days of Dhul-Hijjah, because Ibn ‘Abbas (may Allah be pleased with him) narrated that the appointed days are the first ten days of Dhul-Hijjah.
 ‘Abdullah ibn ‘Umar (may Allah be pleased with him) narrated that the Prophet (peace and blessings of Allah be upon him) said: “There are no days that are greater before Allah or in which good deeds are more beloved to Him, than these ten days, so recite a great deal of tahlil, takbir and tahmid during them.” (Narrated by Ahmad, 7/224; its isnad was classed as sahih (sound) by Ahmad Shakir.) 
Formulas of Takbir
The takbir is as follows:
 Allahu akbar, Allahu akbar, la ilaha ill-Allah, Allahu akbar, wa Lillah il-hamd (Allah is Most Great, Allah is Most Great, there is no god but Allah; Allah is Most Great and to Allah be praise).  
And there are other versions. 
This takbir has become a forsaken Sunnah nowadays, especially at the beginning of these ten days. You can hardly hear it except from a few. It should be recited out loud, so as to revive the Sunnah and remind the heedless. 
It was proven that Ibn ‘Umar and Abu Hurayrah (may Allah be pleased with them both) used to go out to the marketplace during the first ten days of Dhul-Hijjah, reciting the takbir out loud, and the people would recite takbir when they heard them, i.e., the people would remember the takbir and each one would recite it by himself; it does not mean that they recited in unison – that is not prescribed in the Shari`ah. 
Reviving Sunnahs that have been forgotten brings a great deal of reward, as is indicated by the words of the Prophet (peace and blessings of Allah be upon him): “Whoever revives one of my Sunnahs that has died out after I am gone, will have a reward like that of everyone who does it, without that detracting from their reward in the slightest.” (Narrated by Al-Tirmidhi, 7/443; this hadith is hasan (good) because of corroborating reports. )
3 – Doing Hajj and ‘Umrah 
One of the greatest deeds that can be done during these ten days is Hajj to the sacred House of Allah. Whoever Allah helps to go on Hajj to His House and do the rituals in the required manner will – in sha’ Allah – be included in the words of the Messenger of Allah (peace and blessings of Allah be upon him): “An accepted Hajj brings no less a reward than Paradise.” 
4 – Udhiyah (Animal sacrifice) 
One of the righteous deeds that bring a person closer to Allah in these ten days is to slaughter the sacrifice and to look for a good, fat animal and to spend money on it for the sake of Allah. 
Let us hasten to make the most of these great days, before the negligent regrets what he did, and before he asks to go back and receives no response.
And Allah knows best.