Different nations have their own ways of determining the start of Islam's lunar months, including Ramadan and the following Shawal, which marks the start of Eid Al Fitr.
Some rely on astrological calculations, some use giant telescopes, and others keep to the traditional practice of seeing the new moon with the naked eye.
But this week the issue became politicised, with the start of the Eid holiday split between two days in some nations that are suffering conflict.
In Yemen, citizens in areas held by the Houthi rebels were arrested for ending Ramadan on Tuesday, in line with those parts of the country controlled by the internationally recognised government.
The Iran-backed rebels had declared that the holiday was to start on Wednesday.
On Tuesday in Sanaa, the capital held by the Houthis since 2014, security forces were suppressing any sign of celebrations.
The Saudi-owned Al Arabiya television network said as many as 25 people were arrested at a Sanaa mosque during Eid prayers that day.
"Yemenis were never at odds over personal details of their lives like they are now," one citizen wrote on Facebook.
He was commenting on a post entitled: "Sanaa expresses its opposition to Houthi authorities by secretly ending its fast."
In it, the writer spoke of rebel security men and "religious squads" patrolling Sanaa's markets looking for signs of residents celebrating Eid on Tuesday.
"In Sanaa, even the residents of one apartment block are divided, with some fasting on Tuesday and others celebrating," the post said.
Traditionally, many Arab nations followed Saudi Arabia's reading of when a lunar month starts and finishes, but more recently, many nations began making that call for themselves.
The Saudis celebrated Eid on Tuesday but Egypt, Syria and Jordan on Wednesday.
Some residents of countries with divided starts to Eid, as was the case in parts of the Palestinian Territories, were unhappy.
"For these differences to exist within a single nation is shameful and embarrassing, and shows how deep the divisions are in our nation," tweeted Belal Al Hashemi, a Palestinian.
In Sudan, the Transitionary Military Council declared Wednesday to be the first day of Eid.
But civilian protest leaders, continuing their trial of wills, declared that Tuesday was the first day of Eid, quoting Khartoum University astrological experts.
The claimed the military leaders, who have begun an investigation into the deaths of 60 people in a crackdown on a sit-in outside the Defence Ministry, wanted people to fast for another day to make them less likely to protest against the generals.
They called on people to demonstrate after the morning Eid prayers to demand that the generals who removed longtime ruler Omar Al Bashir in April step down.
A similar situation arose in Syria, torn by more than eight years of civil strife.
The top religious judge there announced that Eid began on Wednesday, while the opposition Syrian Islamic Council told followers to celebrate on Tuesday.
In Iraq, differences between the Sunni minority and the Shiite majority over the start of Ramadan and Shawal have over the years meant that followers of each celebrated Eid Al Fitr on different days.
But this became more pronounced after the removal of Saddam Hussein, a Sunni, in the 2003 US-led invasion.
This week the spiritual leader of the Shiites, Grand Ayatollah Ali Al Sistani, issued a statement declaring Tuesday to be the 30th and final day of Ramadan.
The Sunnis, following their own religious authority, celebrated Eid on Tuesday.
It was a little different in Egypt, the most populous Arab nation, with 100 million people.
Authorities declared soon after sunset on Monday that Eid would begin on Wednesday, but reports by media outlets, including some owned by the state, later reported that another attempt at sighting the young moon would be made later that night.
It prompted the office of the chief theologian to issue a statement denying the reports and affirming that Eid would begin Wednesday. The lunar month never runs to 31 days.
The confusion gave rise to a wave of posts on social media that bemoaned the failure by the Muslim nations to agree on a unified method to calculate months.
"Please take another look after you switch the telescope off and back on again," many asked Egypt's authorities.
