Narrated Ibn Abbas:
The Prophet said, "No good deeds done on other days are superior to those done on these (first ten days of Dhul Hijja)." Then some companions of the Prophet said, "Not even Jihad?" He replied, "Not even Jihad, except that of a man who does it by putting himself and his property in danger (for Allah's sake) and does not return with any of those things."
Allah (swt) swears an oath by them, and swearing an oath by something is indicative of its importance and great benefit.
The Prophet (pbuh) testified that these are the best days of this world, and encouraged people to do righteous deeds on these days.
Among the good deeds which the Muslim should strive to do during the first ten days of Dhu’l-Hijjah are:
Fasting. It is Sunnah to fast on the ninth day of Dhu’l-Hijjah, because the Prophet (peace and blessings of Allah be upon him) urged us to do good deeds during this time, and fasting is one of the best of deeds.
Takbeer. It is Sunnah to say Takbeer ("Allahu akbar"), Tahmeed ("Al-hamdu Lillaah"), Tahleel ("La ilaha ill-Allah") and Tasbeeh ("Subhaan Allah") during the first ten days of Dhu’l-Hijjah, and to say it loudly in the mosque, the home, the street and every place where it is permitted to remember Allah and mention His name out loud, as an act of worship and as a proclamation of the greatness of Allah, may He be exalted. Men should recite these phrases out loud, and women should recite them quietly.
Doing more good deeds in general, because good deeds are beloved by Allah and will bring a great reward from Him. Whoever is not able to go to Hajj should occupy himself at this blessed time by worshipping Allah, praying (salaat), reading Qur’an, remembering Allah, making supplication (du’aa’), giving charity, honoring his parents, upholding the ties of kinship, enjoining what is good and forbidding what is evil, and other good deeds and acts of worship.
Sacrifice. One of the good deeds that will bring a person closer to Allah during these ten days is offering a sacrifice, by choosing a high-quality animal and fattening it, spending money for the sake of Allah.
Sincere repentance. One of the most important things to do during these ten days is to repent sincerely to Allah and to give up all kinds of disobedience and sin.
Repentance means coming back to Allah and foregoing all the deeds, open and secret, that He dislikes, out of regret for what has passed, giving it up immediately and being determined never to return to it, but to adhere firmly to the Truth by doing what Allah loves.
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.