On the issue of fasting in regions of abnormally long daylight hours
Question: We live in northern European countries where the day is very long during the summer, and for fasting purposes, it could be twenty hours or more. So, what is ruling for fasting in such locations?
Answer: If the fasting period, at any location, is more than seventeen-and-a-half hours, then the faithful could opt for one of the following:
1. To fast according to the local default times, i.e. observe the fast prior to the fajr time of that location, and break the fast at that location’s Maghrib time.
2. To begin one’s fast prior to the fajr time of that location, and break the fast after a duration equal to the fasting period of cities which have normal daylight hours. For example, the fasting duration of the holy city of Karbala in the peak summertime is about sixteen-and-a-half hours, and to observe the fasting obligation, the faithful in cities such as Stockholm, Oslo, or London, may fast for the duration of sixteen-and-a-half hours beginning from their local fajr time and then break their fast.
So if the local fajr time in London, say, is 03:00, then they may break their fast sixteen-and-a-half hours later at 19:30.
It should be noted that option #2 above is only applicable when the local fasting period is more than seventeen-and-a-half hours. If it is equal or less than seventeen-and-a-half hours, then one may only revert to option #1 above, and fast as per normal.
In the case of [option#2], when breaking the fast during the daylight hours, and having early iftar, the Maghrib and Esha prayers would have to be performed after sunset, at Maghrib time.
[Similarly, if the days are abnormally short, say during the winter, then the faithful must also fast according to the then fasting period of cities of average or normal daylight hours, for example the holy city of Karbala, and may not suffice to the short days of their locations as the fasting period. This ruling applies when the duration of the fasting hours is less than 6 hours.]
For more see Islamic Law (pdf)