What exactly do you mean by "synchronized"? There still are other calendars (e.g. Islamic, Jewish, Persian) and they are used not only for religious purposes.

Also, even in Gregorian calendar some countries have weekend on Friday-Saturday or even Thursday-Friday.

I understood the question as: How did humanity agree that today is Monday, even after the first humans travelled from continent to another? He is asking about sccessfully KEEPING TRACK, even between ancient China & ancient Iraq for example.