In most countries Monday is the first day in weekly calendars.
In some languages, like in Chinese, Monday (xingqiyi) means "first day".

In some countries, for example in the U.S., calendars start with Sundays. But even in the U.S. certain academic calendars start with Monday, since most schools do not have classes on Saturdays and Sundays. Many U.S. plane schedules indicate Monday as Day 1. Many work calendars also start with Monday.