Short answer
You fast from true dawn (fajr) to sunset where you are, however long the day, as long as day and night both happen within 24 hours. If fasting that long would genuinely harm you, the rules for illness apply. Where true dawn cannot be seen, such as the far north in summer, scholars estimate the times, and you follow a reliable local timetable.
The fast runs from true dawn to sunset by local times, even if the day lasts 18 to 20 hours. Where true dawn cannot be told apart because twilight lasts all night, the Hanafi jurists discussed this case and later scholars estimate fajr from the nearest place or latitude where it is clear. Where the sun does not set for days, the timings of the nearest place with a normal day are used.
The four schools agree on this.
Explanation
Allah set the fast from the white thread of dawn until night. If the sun rises and sets where you live, those are your times, even in a long northern summer. Muslims in the UK, Canada and Scandinavia have fasted long days for generations, and contemporary scholars of the four schools agree in principle on this.
When dawn cannot be seen. In parts of the UK, northern Europe and Canada, the evening twilight never fully disappears in summer, so true dawn is hard to fix. Ibn Abidin discussed a similar case for the people of Bulghar. The Prophet ﷺ, speaking of the long days at the time of the Dajjal, told the companions to estimate the prayer times. On this basis scholars use estimates such as the nearest latitude where dawn is clear or a share of the night.
- Follow one reliable timetable from your local masjid or scholars, and stick to it.
- Do not switch between timetables to find the shortest fast.
- Eat a proper suhoor and drink well between iftar and fajr.
If it would harm you. A person who would genuinely be harmed by very long days, such as an elderly person or someone with a health condition, takes the ruling of the sick: they may leave the fast and make it up later, for example in the short winter days. Ordinary hunger and thirst are not harm. See fasting while sick and the Ramadan fasting rules.
The evidence
Quran, Surah Al-Baqara 2:187
أُحِلَّ لَكُمْ لَيْلَةَ ٱلصِّيَامِ ٱلرَّفَثُ إِلَىٰ نِسَآئِكُمْ ۚ هُنَّ لِبَاسٌۭ لَّكُمْ وَأَنتُمْ لِبَاسٌۭ لَّهُنَّ ۗ عَلِمَ ٱللَّهُ أَنَّكُمْ كُنتُمْ تَخْتَانُونَ أَنفُسَكُمْ فَتَابَ عَلَيْكُمْ وَعَفَا عَنكُمْ ۖ فَٱلْـَٰٔنَ بَٰشِرُوهُنَّ وَٱبْتَغُوا۟ مَا كَتَبَ ٱللَّهُ لَكُمْ ۚ وَكُلُوا۟ وَٱشْرَبُوا۟ حَتَّىٰ يَتَبَيَّنَ لَكُمُ ٱلْخَيْطُ ٱلْأَبْيَضُ مِنَ ٱلْخَيْطِ ٱلْأَسْوَدِ مِنَ ٱلْفَجْرِ ۖ ثُمَّ أَتِمُّوا۟ ٱلصِّيَامَ إِلَى ٱلَّيْلِ ۚ وَلَا تُبَٰشِرُوهُنَّ وَأَنتُمْ عَٰكِفُونَ فِى ٱلْمَسَٰجِدِ ۗ تِلْكَ حُدُودُ ٱللَّهِ فَلَا تَقْرَبُوهَا ۗ كَذَٰلِكَ يُبَيِّنُ ٱللَّهُ ءَايَٰتِهِۦ لِلنَّاسِ لَعَلَّهُمْ يَتَّقُونَ ﴿١٨٧﴾
It has been made permissible for you the night preceding fasting to go to your wives [for sexual relations]. They are clothing for you and you are clothing for them. Allah knows that you used to deceive yourselves, so He accepted your repentance and forgave you. So now, have relations with them and seek that which Allah has decreed for you. And eat and drink until the white thread of dawn becomes distinct to you from the black thread [of night]. Then complete the fast until the sunset. And do not have relations with them as long as you are staying for worship in the mosques. These are the limits [set by] Allah, so do not approach them. Thus does Allah make clear His ordinances to the people that they may become righteous.
The fast runs from true dawn to sunset wherever you are.
Sahih Muslim 2937
Sahih... أتكفينا فيه صلاة يوم قال لا اقدروا له قدره ...
"They asked whether the prayers of one day would suffice on that day as long as a year. He said: No, estimate its measure."
Where the normal signs of time fail, times are estimated.
Where the ruling is recorded
- Radd al-Muhtar (Ibn Abidin), Kitab al-Salah, chapter on prayer times, the issue of one who does not find the time of Isha. The discussion of the people of Bulghar and estimating times where the signs do not appear.
- Fatawa Mahmudiyya, Kitab al-Sawm. Fasting in lands with very long days follows local dawn and sunset.
This answer is pending review by the scholars on our team. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.