"The Prophet, upon whom be peace, would fast on the ninth of Dhu'l-Hijjah, the day of Ashura and three days in every month."
Recorded by Ahmad, Abu Dawud, An-Nisa'i and others
Imam An-Nawawi said that fasting in these ten days is extremely recommended.
I remember years ago brothers saying
Imam An-Nawawi has no proof.
Then I came across the above hadith last night.
http://sunnahonline.com/library/hajj...hul-hijjah-the
DHU'L-HIJJAH 1437H (2016CE): According to the decision of the High Judiciary Council of Saudi Arabia, the 1st of Dhu'l-Hijjah will be on Saturday 3 September 2016. The Muslims performing Hajj will be in 'Arafah on Sunday 11 September 2016 (9 Dhu'l-Hijjah 1437), and 'Eid ul-'Adha shall be on Monday 12 September 2016 (10 Dhu'l-Hijjah 1437).