Why do calendars start on Sunday?

Because Sunday is the first day of the week, and not Monday as some people think.

Following the Dark Ages, secular publishers of calendars adopted the form used by the church and monasteries.

(See the related question below.)
Many calendars begin with Sunday because may people treat Sunday as the first day of the week. Monday, is just the first day of the working week.

However, some people follow the Biblical implication that, as the Christian Sabbath, Sunday was the last day of the week, for them some calendars begin with Monday.