Fathwa - Fasting on the eleventh day of Muharram thinking that it is ‘Aashooraa’

Question
What is the ruling of fasting on a day thinking that it is the tenth
of Muharram according to the calendars found in bookshops but, in the
middle of the day, I discovered that it is the eleventh day of
Muharram?
Answer
All perfect praise be to Allaah, The Lord of the worlds. I testify
that there is none worthy of worship except Allaah, and that Muhammad,
is His slave and messenger.
You should have checked and asked about the timewhen the moon was
sighted in a Sharee'ah-approved manner. You should not have depended
only on the calendar that may have been right or wrong. What counts is
a Sharee'ah-approved moon sighting.
If you did not fast on the tenth day thinking that itis the ninth day,
then it isan act of the Sunnah thatyou missed and, due to this, you
also missed the day of 'Aashooraa' because it is the tenth day of
Muharram. You are rewarded, Allaah Willing, for your intention and
sincere desire to fast on the day of 'Aashooraa'. The Prophet,
sallallaahu 'alayhi wa sallam, said: "When a slave falls ill or
travels, then he will receive a reward similar to that which he gets
for good deeds he used to perform at home while in good health."
[Al-Bukhaari]
You are also rewarded for your voluntary fastingon the eleventh day.
Allaah The Almighty never Wastes the reward of he who does good deeds.
From here on, youshould seek (a confirmation of) the moon being
sighted to avoid making such a mistake.
Allaah Knows best.