It's only "different" from the Gregorian calendar that most folks follow... which also is based on lunar cycles, but with all sorts of adjustments thrown in.
On other calendars, the significant days always fall on the same day every year, just as they do with the Gregorian calendar.
Uh, except for Thanksgiving... and Easter... and Memorial Day... and Labor Day...

...Or am I wrong?

Anyway...a tranquil Ramadan to all Muslims out there;
Salaam Malakim.
Difference is that the islamic calendar originally didnt need seasonal adjustments (like the georgian calendar).
If mr Bede and Caesar hadn't take action, Christmas would have been in mid summer.
I've never been able to understand just when the Ramadan starts... It's different every year... enlighten me please...
To make it even more complicated... it differs between countries too.
There are three systems (and variations on them too)
[*]precalculated start and end dates
[*]moon sightings in the country itself
[*]moon sightings in Mecca
Precalculated start date and ending on Mecca moon is a quite common one.
Turkey uses calculated dates, and sometimes is a day ahead at the start.