The reason is it is what is known as a movable feast, much like the christian Easter. The way it is decided is they first interpret the moon cycle for Ramadan when the moon is in the right phase at a particular point and coincides with a particular date of the year Ramadan is called. The reason for the 30 days is it takes that long for the moon to complete one full cycle and Ramadan ends when this cycle is complete and you have Eid Al Fitr...the working out out Eid Al Hadr is calculated, as I was informed to be 60 days + 10 from the end of Eid Al Fitr. Given the Islamic calendar does not coincide with the Gregorian calendar as it is shorter, the timings for these will always differ from year to year.

"Your born, You Live, You Die, given this premise, one can conclude since we have no control over when we are born and when we die, the only thing that matters to us should be how we live, simple really?" Mis-Cat to her philosophy Lecturer.