It has now been decided that India will celebrate Eid on Saturday, 22 April, while some countries around the world have already started celebrating the festival on Friday. In India, every year, there is a lot of discussion and deliberation about the moon sighting for Eid.
To attain a better understanding of this debate, let's look at the conference that was organized from 28-30 May 2016 in Istanbul, Turkey, which tried to determine the new Hijri lunar month, in accordance with the rules of Islam with respect to astronomy.
The Hijri calendar is based on the rotation of the moon around the Earth, and a lunar month is completed with the completion of one rotation of the moon. This calendar begins with the year of the Prophet's migration from Mecca to Medina, but its regularization began 17 years later. It should also be noted that after the death of the Prophet, the second caliph of the Muslims, Hazrat Umar Ibn Al-Khattab, after consulting the Companions, adopted the Hijri calendar, which was previously known as the Arab calendar.
Now, back in 2016, the International Beginnings of Qamri Months and Hijri Calendar Unity Congress was an event in which famous Islamic scholars from Turkey, Qatar, Jordan, Saudi Arabia, Malaysia, the United Arab Emirates, Morocco, America, and fifty European countries gathered.
This conference came to be known as the International Hijri Calendar Union Congress, and having factored in the differences among Muslims globally, laid out the rules based on which they would celebrate Eid ul-Fitr and Eid ul-Adha on the same day.