If there is an error in the moon sighting, and it is proven that we did not fast on one day of Ramadan, do we have to make it up?

Praise
be to Allah

If it is proven
by shar‘i methods that the Muslims made a mistake with regard to defining
the beginning or end of Ramadan, then they must make up for this mistake and
make up the day of Ramadan that they did not fast.

Such a mistake
may be established by a number of shar‘i methods, including the following:

1.    

If they completed Sha‘baan with thirty days, then a
trustworthy person comes and testifies that he saw the new moon on the night
before the thirtieth of Sha‘baan, and the judge accepts his testimony.

2.    

If they fasted Ramadan with twenty-eight days, then they saw
the new moon of Shawwaal.

If that is
proven, then they must make up one day for the day concerning which they
were mistaken.

Shaykh ibn Baaz
(may Allah have mercy on him) said:

It is proven in
many saheeh hadiths from the Messenger of Allah (blessings and peace of
Allah be upon him) that the month cannot be less than twenty-nine days. If
it is proven by means of shar‘i evidence that Shawwaal has begun, when the
Muslims have only fasted twenty-eight days, then this means that they did
not fast the first day of Ramadan, so they must make it up, because it is
not possible for a month to have twenty-eight days; rather it can only be
twenty-nine or thirty days.

Shaykh al-Islam
Ibn Taymiyyah said in vol. 25 of his Fataawa (p. 154-155) that this
happened during the time of ‘Ali; they fasted twenty-eight days. He ordered
them to fast the day that they had missed, and to complete the month as
twenty-nine days.

End quote from
Majmoo‘ Fataawa ash-Shaykh Ibn Baaz (15/158).

This happened in
the land of the Two Holy Sanctuaries in 1404 AH, and the  Standing Committee
for Issuing Fatwas issued a fatwa stating that it was obligatory to make up
the day which they had not fasted at the beginning of Ramadan.

It says in
Fataawa al-Lajnah ad-Daa’imah li’l-Iftaa’ (10/122):

The sighting of
the new moon for Ramadan 1404 AH was not proven according to shar‘i
conditions by the responsible parties in the Kingdom of Saudi Arabia except
on the night before the Thursday, so they issued instructions to complete
Sha‘baan as thirty days, in accordance with the saheeh hadiths to that
effect, and announced that Ramadan would begin that year on the Thursday.
Then they looked for the new moon of Shawwaal 1404 AH and its sighting was
proven by them on the night before the Friday. So they announced that Eid
al-Fitr 1404 would be on the Friday. Thus they fasted twenty-eight days, and
the lunar month cannot be twenty-eight days; rather it can only be
twenty-nine days sometimes and thirty sometimes, as is proven in the saheeh
hadith.

This mistake
proved that the beginning of Ramadan had been delayed, so they announced
that and instructed people to make up the day that they had not fasted at
the beginning of the month, so as to discharge their duty and do what is
right. End quote.

The Standing
Committee for Academic Research and Issuing Fatwas

Shaykh ‘Abdullah
ibn Qa‘ood, Shaykh ‘Abdullah ibn Ghadyaan, Shaykh ‘Abd ar-Razzaaq ‘Afeefi,
Shaykh ‘Abd al-‘Azeez ibn ‘Abdillah ibn Baaz

But if the
mistake cannot be proven by shar‘i means; rather it was based on
astronomical calculations or on what some people thought, then this is of no
significance and shar‘i rulings cannot be based on it.

And Allah knows
best.