Try this:
$('#datepickerfield').datepicker({
constrainInput: true,
minDate: new Date(),
showOn: 'button',
autoSize: true,
altFormat: 'yy-mm-dd',
beforeShowDay: $.datepicker.noWeekends,
firstDay: 1
})
or this
var firstDay = $('.selector').datepicker('option', 'firstDay');
$('.selector').datepicker('option', 'firstDay', 1);