The first day of the month of Shawwal on the lunar calendar is called Eid al-Fitr, which is marked when the first crescent of the new moon is sighted.
In many Muslim countries, moon-sighting committees are set up to confirm the start of the month of Shawwal.
According to Muhammad Shawkat Awda, head of the Islamic Moon-Sighting Initiative, in most Muslim countries the moon will set before the sun on Monday and, therefore, the moon cannot be sighted.
On Monday, sighting the moon in Asia, Oceania, Europe and North Africa will be impossible, he said, adding that it can be only seen by telescope in parts of South America.
That is why, he said, the European Council for Fatwa and Research has announced Tuesday as Eid in those regions.
But elsewhere in the world, the beginning of Shawwal and Eid al-Fitr will most probably fall on Wednesday, he went on to say.