Where can I change Datepicker?

I discovered that calderaforms uses uses jQuery UI’s datepicker, which works great.
We need to make certain days(1,15,30,31) unavailable. I’m sure even pro can’t do this.

I know I should create a datepicker2 before I modify it with this code from jsfiddle:
example here: https://jsfiddle.net/arul4/hru9bfL5/1/

$('#firstPaymentDate').datepicker({
    //startDate: new Date(),
    beforeShowDay:  function (date) {
        //getDate(); //returns the day (0-31)
        if (date.getDate() == 29 || date.getDate() == 30 || date.getDate() == 31) 
        {
            return [false, '', 'Unavaliable'];
        }
        return [true, ''];
    },
    format: "dd-mm-yyyy",
    todayHighlight: true
}).datepicker('setDate', now).attr("readonly", "readonly");

But could someone tell me how I can intercept the standard calderaform function call to use this? Or send me somewhere to make this happen?

Thanks.

Paul

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