A Muslim month is started after sight of the new moon by the necked eye.
Most of the countries use the method of moon sighting to celebrate the special days. Some occasion dates are determined only after sight of the new moon, such as Eid Al Fitr.
However, a month can’t be longer than 30 days. So, if a new moon is not seen on 30th day, a new month will start automatically on the following day.
For this reason, the Muslim special days are not celebrated on the same day in all countries of the world but this difference is only of 1 day maximum.
Each Islamic state declares the sight of the moon in their territory officially to determine the special day.
Usually a committee is there to observe the moon and declare the sight of the moon officially.
In Saudi Arabia, Umm Al Qura calendar is also used exclusively for administrative purpose which is based on astronomical calculation.
The beginning of a Hijri month is determined by this calendar and so do the special days.
This parameter also has undergone several modifications over the past decades.
The Islamic Society of North America, the Fiqh Council of North America and the European Council for Fatwa and Research announced in 2007 that they will henceforth use the parameter of Umm Al Qura calendar to determine well advance the beginning of all lunar months and related religious observance days.