Indeed it is a great favour and blessing from ALLAH that He has made for His righteous servants periods of time within which they can increase in good deeds to attain great rewards and one of these opportunistic periods is the first ten days of Dhul-Hijjah (the final month of the Islamic Calendar, in which the hajj is performed).
The Messenger of ALLAH, peace be upon him, said:
"There are no deeds as excellent as those done in these ten days." They (the companions listening) said, "Not even Jihâd?" He, peace be upon him, said, "No, not even Jihâd except a man who goes forth endangering his life and wealth and does not return with anything." [Recorded by al-Imâm al-Bukhârî]
" … and remember the name of ALLAH in the appointed days." [Al-Qur'ân 2:203]
Ibn Abbâs and Ibn Kathîr said this means in the ten days of Dhul-Hijjah.
The Messenger of ALLAH, upon whom be peace, said:
"There are no other days that are as great as these in the sight of Allah, the Most Sublime. Nor are there any deeds more beloved to Allah then those that are done in these ten days. So increase in tahlîl (to say lâ illaha illallah), takbîr (to say allahu akbar) and tahmîd (to say alhumdulillâh)." [Reported by at-Tabarânî in al-Mu'jum al-Kabîr]
"The most apparent reason for the ten days of Dhul-Hijjah being distinguished in excellence is due to the assembly of the greatest acts of worship in this period, i.e. salawât (prayers), siyâm (fasting), sadaqah (charity) and the hajj (pilgrimage). In no other periods do these great deeds combine."