date_field_start_date.js

<script>
jQuery(document).ready(function($) {
    "use strict";

    /* change the calendar start day when date field receives foucus */
    $('#field_your_date_field_id').on('focus', function() {

        /* apply the radio button's value to the firstDay option */
        $(this).datepicker('option', 'firstDay', 1);
    });
 });
</script>