Question: Which days of the year fasting is haram for Muslims?

Answer: Throughout the year there are five days which are forbidden, those are: two eid days and 11, 12, 13th day of Zul-Hijjah.  It is narrated in the hadith by Abdullah Ibn Huzafa r.a who said: “Do not fast in these days, as these days are for eating, drinking and having (physical) relation with spouse.”

References:  [1] Sunan al Kubra (Nasai): 2882 [2] Ahmed: 15735 [3] Daru Qutni: v-2, p-212 [4] Al-Bahrur Raiq vol-2:258 [5] Ad Durrul Mukhtar vol-1:381