Sample: dhtmlxCalendar Changing week start day dhtmlxCalendar main page
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This sample illustrates that you can control the start day of week by using 'weekstart' property.
Week starts with MondayWeek starts with Sunday

 
    <script>
 
        mCal1 = new dhtmlxCalendarObject('dhtmlxCalendar2', false, {isYearEditable: true});
        ...
        mCal1.options.weekstart=1;//start from Monday
        //mCal1.options.weekstart=0; //Start from Sunday
        mCal1.draw();
</script>

Also, starting day for the week can be defined in overal language settings:
 
//sets Sunday and Saturday as weekend. Weeks starts with Monday
{
    ...
    weekend:    [0, 6],
    weekstart:    1,
}