The official start of Ramadan is usually not determined until the day before, even though astronomical calculations can predict the approximate beginning of the holy month. Pictured: A crescent moon appears over a mosque after sunset in Karachi, Pakistan, on June 29, 2014. Reuters/Akhtar Soomro

Judging the start of the holy Muslim month of Ramadan can be a tricky task. Every year, Ramadan shifts a bit earlier, beginning and ending on a different date; even its preset dates are subject to change from country to country. Here's everything you need to know about Ramadan 2015 -- when it's slated to start, why that can change and when we'll all officially know that Ramadan has begun.

Ramadan, the ninth month of the Islamic calendar, is expected to begin this year the night of Thursday, June 18, and end the evening of Friday, July 17. Even though astronomy can generally predict the lunar cycle, Muslims continue to depend on the tradition of looking at the sky to confirm the start of the holy month, in part because sightings can vary depending on where they live around the world.

The day after a new, waxing crescent moon -- a sliver known as a "hilal" -- appears in the night sky is the first day of Ramadan. Some Muslims observe the moon locally, while others will defer to the authority of a moon-sighting committee in the holy city of Mecca, Saudi Arabia, Gulf News explains. So we will not know when Ramadan officially begins until one night before.

Similarly, Ramadan ends when the cresent of a new moon appears -- and that, too, requires scanning the skies. It's a tradition bordering on ritual, one that some Muslims have objected to on the ground that if modern astronomy can so precisely predict the moon's waxing and waning, observers of the holy month should be allowed to plan ahead based on those very calculations. But that dispute remains unresolved.

Ramadan is based on a lunar calendar, which means that its timing shifts every year about 11 days earlier than the previous Ramadan. Ramadan, marked by fasting and reflection, is an important celebration in the Muslim calendar because Muslims believe it to be the month that Allah revealed the Quran, the holy book of Islam, to the Prophet Muhammad.