Checkbox checked property not setting checkbox in Form 7

This is my code, it hides and shows the checkboxes just fine, only the boxes are not checked or unchecked. I use this in the WordPress Form 7 plugin.

$("#haaldatum").change(function() {
            var curDateH = $("#haaldatum").datepicker('getDate');
            var dayNameH = $.datepicker.formatDate('DD', curDateH);
    if (dayNameH == "zaterdag") {
        $("#haaltijdzat").show().prop( "checked", true );
        $("#haaltijd").hide().prop( "checked", false );

    }
        else{
        $("#haaltijdzat").hide().prop( "checked", false );
        $("#haaltijd").show().prop( "checked", false );

    }