I know it sounds a bit like a silly question. Every culture in the world follow a different Calendar, for instance:
Christians follow a solar calendar following the birth of Jesus Christ (2014. AD currently running)
Muslims follow a lunar calendar (Hijri) based on the birth of Prophet Mohammed.
Hindus follow a luni-solar calendar called Vikram Samvat (among many other versions).
Now the number of months and days in a month are obviously different in each of this calendar. However, how come all calendars happen to agree on the seven day week system (For example, today is a Friday in all calendars) ?
Looks like it should be obvious, I know. But still what makes it possible that they agree on the week and still differ in various other calculations?