According to Abu Abbas that the Prophet (PBUH) said: No deeds during the year days are better than the deeds in the ten days of Dul Hajj. They said: Even Jihad in the way of Allah? The Prophet (PBUH) said: Even Jihad except a man goes for Jihad with his wealth and does not return back with any things. Narrated by Bukhary.
These are the days in which the fifth pillar of Islam, Hajj, falls and where delegates of Hajj come from different places to meet. Allah Almighty says in Sura Al Baqarah V:198: Then when you leave, Arafat, member Allah at the mash ar-il-Haram. And remember Him as He has guided you and verily, you were, before, of those who were astray.
And the best day among the 10 days of Dul Hajj is the day of Arafat. That is why the Prophet (PBUH) advises those who are not going for Hajj, to fast this day. According to Abu quatada that when the Prophet (PBUH) was asked about fasting the day of Arafat He said: It clears the sins of current and previous years. Narrated by Muslim. The Prophet (PBUH) also said: Verily, there is no day where the number of people whom Allah will be free from Hell fire, is higher than the day of Arafat. Narrated by Muslim.
According to Abu Abbas that the Prophet (PBUH) said: That Allah Almighty took the pledge from the children of Adam on the day of Arafat and said to them: Am I not your Lord? They said yes we testify, lest you should say on the Day of Judgment. ?Verily we have been unaware of this? narrated by Ahmed. This indicates as if all people recognize their lord on this day and such recognition remains with them unless it is damaged by bad environment.
These days are seasons for purifying our selves and rectifying our conditions therefore, Muslims should not miss such opportunities.