Question Summary:
When is Muharram Day? The day to fast?

Question Detail:

When is muharram day? The day to fast.

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
Muharram is the first month of the lunar calendar. The 10^th of Muharram is the day of Ashura when it is Sunnah to fast.
Consider the following hadith:
أَنَّ عَائِشَةَ رَضِيَ اللهُ عَنْهَا، قَالَتْ: كَانَ رَسُولُ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ يَأْمُرُ بِصِيَامِهِ قَبْلَ أَنْ يُفْرَضَ رَمَضَانُ، فَلَمَّا فُرِضَ رَمَضَانُ، كَانَ مَنْ شَاءَ صَامَ يَوْمَ عَاشُورَاءَ وَمَنْ شَاءَ أَفْطَرَ
'A'isha (Radiallahu Anha) reported that the Quraish used to fast on the day of 'Ashura in the pre-Islamic days and the Messenger of Allah (Salallahu Alayhi Wasallam) also observed it. When he migrated to Madina, he himself observed this fast and commanded (others) to observe it. But when fasting during the month of Ramadan was made obligatory he said: He who wishes to observe this fast may do so, and he who wishes to abandon it may do so. (Muslim-1125)
Kindly refer to the following detailed article for more details: http://askimam.org/public/question_detail/30994
And Allah Ta’āla Knows Best                                       
Ismail Desai,
Darul Iftaa
Durban, South Africa
Checked and Approved by,
Mufti Ebrahim Desai.