How to set up Pikaday widget with week begin from monday?

I know for if need set “firstDay” prepority of Pikaday js object (first day of the week (0: Sunday, 1: Monday, etc)).
But I don’t know where i can do it…

<script src="pikaday.js"></script>
<script>
    var picker = new Pikaday({ firstDay:1, field: document.getElementById('datepicker') });
</script>