Muslims follow lunar calendar and not solar calendar. In lunar calendar the year is 11 to 12 days shorter than the solar year. So one cannot say if the days and dates of Eid-Ul-Fitr for the year 2016 would be similar to that of the year 2015
The beginning of the Ramadan, ninth month of Islamic calendar is also decided by the sighting of the Crescent moon (hilal) This Sighting of the hilal marks the beginning of the Ramadan. If hilal is seen at night then the next day is the first day of Ramadan and fasting.
On the similar lines when the hilal is visible again at the end of the month, it is the time to celebrate Eid ul Fitr (break the fast). When the hilal is sighted the beginning of the new month Shawwal, the tenth month of Islamic calendar and the time to celebrate Eid-ul-fitr.
Visibility of the crescent moon can be affected by a number of factors and so it is recommended that number of observers should be involved to sight a correct Crescent moon specially in cases where naked eyes are used to detect the Eid moon.
Factors affecting the sighting of the crescent moon:
Concept of international lunar date line (ILDL) has been developed by Professor Ilyas in Malayia. ILDL forms a curved line on a world map, which separates areas (west of line) where there are greater chances of sighting moon in the beginning of the lunar month along with the areas where the crescent moon is likely to be seen (east of line). The position of ILDL changes from month to month.
On the similar lines Dr.Monzur Ahmed has developed a computer program called Mooncalc, which makes use of ILDL and creates world map to show where the crescent is likely to be seen for the first time. The position, age, phase, orientation, appearance and visibility of the moon for any date, time and location on earth can be find out making the use of ILDL..