++ [ originally posted by Majed
] ++
Just to add what La_dynamite said, Ramadhan is the 9th month of the Lunar calendar, which is around 11 days shorter than the solar calendar year. Since the 4 seasons are always going to start at the same time every solar year, that's not true about the lunar year because the year is shorter. So every year Ramadhan starts around 11 days earlier than the solar date that it started the year before. That goes for all the months.
They way we know that a lunar month started is when we see the moon in it's cresent form after it was dissappeared. The prophet Mohammed PBHU said to fast at the sight of the moon (begining of the month of Ramadhan). In some countries, they aren't able to see the moon yet so the month of Ramadhan starts a day later. It all evens out eventually for the rest of the months.
To add to your confusion

, in many Islamic countires, the official calendar is not the Gregorian Calendar. Those countries use the Hijri calendar. Todat is 30/8/1425 h. The hijri calendar is lunar and it is marked by the Prophet Mohammed's journey to Al-Madinah 1425 years ago (and that another story).