Ramadan is expected to start on May 27, 2017, and last until June 24, 2017.
Eid will then be on June 25, 2017, as the next month (Shawwal) begins.
Why does the date vary?
The Islamic calendar is based on the cycle of the moon, but the Gregorian calendar largely used in the western world is based on the sun.
Because the two calendars don’t align exactly, the Islamic dates move back by 11 days a year.
Ramadan is a month of fasting. Muslims must abstain from food, drink, smoking, sex and sinful behaviour each day, between an early-morning meal (suhur) and a night-time meal (iftar).
It was particularly tough in 2016 because it was in the period leading up to (and including) the longest day of the year.
Because the fast takes place from early in the morning (a couple of hours before sunrise) to sunset, it has meant long periods of around 19 hours without food and drink.
In 2017, it won’t include the longest day but it will still cover a period of extended daylight hours, so it will still be quite a challenge.
Some organisations and communities base their calendar on the sighting of the very first crescent (hilal) of the new moon, using the naked eye.
But others use astronomical charts, and these different methods can lead to variations of one, two or three days in the dates observed by different Muslim communities around the world.
In Saudi Arabia, the Institute of Astronomical & Geophysical Research of the King Abdulaziz City for Science and Technology (KACST) uses modern science to work out the Umm al-Qura calendar, which is used by the country’s government for setting the date of its policies, events and other civic matters.
That calendar is also used by the Islamic Society of North America (ISNA), the Fiqh Council of North America (FCNA) and the European Council for Fatwa and Research (ECFR).
In contrast, the religious authorities in Saudi Arabia still base their decisions on the first sighting of the lunar crescent but they do consider amateur reports from casual observers.
However, astronomers claim there have probably been many wrong decisions in the past that were most likely based on false sightings of a bright star or planet (such as Venus) or even the vapour trail from a plane.