As far as Islamic Sharia is concerned, a Muslim should start and end fasting and celebrate `Eid according to the country he or she is living in when starting or ending fasting. However, if because of the difference in moon sighting the fasting days are less than 29 days, then he or she — after celebrating the `Eid with people — has to make up for the day (s) he or she missed.
Concerning this, the late prominent Saudi scholar Sheikh Ibn `Uthaymeen (may Allah have mercy upon him) said,
If a Muslim travels from one country to another where the moon sighting is different, the basic principle to be followed is that he or she should start and end the fast according to the country he or she is in when it is established that the month has ended. But if that means that he or she has fasted for less than 29 days, then the number of fasting days should be completed, because the lunar month cannot be less than 29 days. This principle is based on the words of Prophet Muhammad (peace and blessings be upon him): “When you see it (the new moon) then fast, and when you see it then break the fast,” and “The month is [or no less than] 29 days, so do not fast until you see it, and do not stop fasting until you see it.” And in the hadith of Kurayb, it says that Umm Al-Fadl sent him to Mu`awiyah in Syria, and Kurayb told Ibn `Abbas (may Allah be pleased with him) that the people had seen the new moon of Ramadan on Friday night in Syria. Ibn `Abbas said, “But we saw it on Saturday night, so we will carry on fasting until we complete 30 days or until we see it.” Kurayb said, “Isn’t the sighting and fasting of Mu`awiyah sufficient for you?” He said, “No; this is what the Messenger of Allah (peace and blessings be upon him) enjoined upon us.”
Following are some examples that explain this principle:
1. A Muslim moves from a land where people started fasting on Sunday to a land where people started fasting on Saturday and these broke the fast on Sunday after 29 days of fasting. He or
she should break the fast with them but has to make up for one day later on.
2. A Muslim moves from a land where people started fasting on Sunday to a land where people started fasting on Monday and these stopped fasting on a Wednesday after 30 days of fasting. He or she should carry on fasting with them even if that means he or she fasts more than 30 days, because he or she is in a place where the new moon has not been sighted, so it is not permissible for him or her to break the fast.
This is similar to the situation if he or she travels while fasting from a place where the sun sets at 6 p.m. to a land where the sun does not set until 7 p.m. — he or she should not break the fast until the sun sets at 7 p.m., because Allah says,
[Then strictly observe the fast till nightfall and touch them not, but be at your devotions in the mosques. These are the limits imposed by Allah, so approach them not. Thus Allah expounds His revelations to humankind that they may ward off (evil).] (Al-Baqarah 2:187)
3. A Muslim moves from a land where people started fasting on Sunday to a land where people started fasting on Monday and these stopped fasting on Tuesday after 29 days. He or she should break the fast with them and the people’s fasting will be 29 days while his or hers will be 30 days.
4. A Muslim moves from a land where people started fasting on Sunday and stopped fasting on Tuesday after 30 days, to a land where people started fasting on Sunday and stopped fasting on Monday after 29 days. He or she should break the fast with them, and he or she does not have to make up one day, because he or she has completed 29 days.
The evidence that he or she has to break the fast in the first example is that the new moon has been sighted, and the Prophet (peace and blessings be upon him) said, “When you see it [the moon], stop fasting.” The evidence that he or she has to make up for one day is the words of the Prophet (peace and blessings be upon him), “The month is 29 days,” so it cannot be less than 29 days.
The evidence that he or she may continue to fast for more than 30 days in the second example is the words of the Prophet (peace and blessings be upon him), “When you see it, then stop fasting.” So ending the fast is connected to the sighting of the moon: If it is not sighted, then that day is Ramadan in that place, so it is not permissible to break the fast.
The ruling in the third and fourth examples is obvious. This is what we [Ibn `Uthaymeen] think is the ruling with regard to this issue, and it is based on the most correct view: The ruling varies according to the moon sighting.