Whether you prefer your monthly calendar's weeks to begin on Sunday or Monday has a lot to do with where you live.
In the US, monthly calendars' weeks usually start on Sunday. Many people in the US don't like monthly calendars that start on Mondays because they are used to the Sunday start ones and find Monday-starting weeks confusing. I grew up with all monthly calendars starting their weeks on Sunday.
Elsewhere in the world, monthly calendars' weeks usually start on Monday, which I prefer. It keeps the weekend together for easier planning, and I like that my weekly planner pages directly match up with the corresponding week on my monthly calendar.
Quo Vadis blog had a post on this not long ago, and even though most of their readers are in the US there were still mixed replies.
Which do you prefer, Sunday or Monday start? Please vote in the poll in the sidebar!