Among the great seasons of worship are the first ten days of Dhu’l-Hijjah, which Allaah has favoured over other days.
It was narrated from Ibn ‘Abbaas (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “There are no days on which righteous deeds are more beloved to Allaah than these ten days.” They said: “Not even jihad for the sake of Allaah?” He said: “Not even jihad for the sake of Allaah, unless a man goes out himself for jihad taking his wealth with him and does not come back with anything.” Narrated by al-Bukhaari, 2/457
It was also narrated from Anas (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “There is no deed that is better before Allaah 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 Allaah?” He said: “Not even jihad for the sake of Allaah, unless a man goes out himself for jihad taking his wealth with him and does not come back with anything.” Narrated by al-Daarimi, 1/357; its isnaad is hasan 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 Ramadaan. But the last ten nights of Ramadaan are better than the first ten nights of Dhu’l-Hijjah, because they include Laylat al-Qadr, which is better than a thousand months. See Tafseer Ibn Katheer, 5/412.
(Source: Islam QA)