Think I've answered my own question....unless there is a better way
private void sfDataGrid1_EditingControlShowing(object sender, Syncfusion.WinForms.DataGrid.Events.DataGridEditingControlShowingEventArgs e)
{
if (e.Column is GridDateTimeColumn)
{
var ctrl = (Syncfusion.WinForms.Input.SfDateTimeEdit)e.Control;
ctrl.MonthCalendar.FirstDayOfWeek = DayOfWeek.Monday;
}
}