Definitely start it on monday. Having calendars that start on sunday comes from Christians starting their week each week with a gathering to celebrate christ’s resurrection (I think they wanted to distinguish themselves from Jews, who celebrate the sabbath on saturday; for Jews sunday is also the first day of the week, and their weekly gathering comes at the end of the week). For most people, even in the US, the church schedule no longer determines the rhythm of their lives, and it makes more sense to start calendars on monday at the beginning of a typical workweek, the way any weekly pocket planner/calendar does, and the way all the monthly calendars do in Europe and elsewhere (counting weeks starting on a monday is even the ISO standard way to count weeks). Then the weekend is properly lumped together at the end of the week instead of being split in half.
