It is not a mistake that in all Calendars on this site Monday is held to be the first day of the week, by default. It is for a few reasons. ISO 8601 defines Monday as the first day of the week, making Sunday the seventh. Moreover, in most countries in the world Monday refers to the first day of the week. Finally, Saturday and Sunday are commonly called the weekend. As Sunday is weekEND, it cannot be the beginning of the week, logically. Also, the week cannot have two ends and no beginning like a piece of string, because it is a quantum of time. Any quantum of time always has its beginning and its end.

However, if it is a tradition in your country to hold another day to be the first day of the week, you can use «Settings window» on this web-site to create a kind of yearly or monthly calendar that you want.