need to override function in jquery.ui.datepicker.js

I haven’t tested this, but this should work and don’t forget to replace the jQuery selector with your own:

$(function() {
 if(typeof jQuery.datepicker === 'function'){
    $( ".your_date_picker_field_selector" ).datepicker({
        beforeShowDay: function (t) {
            var e = t.getDay();
            return [e == 3, ""]
        }
    });
  }
});

Updated: Since, you are using contact form 7 datepicker addon, use filter to customize the meeting date field js.

add_filter( 'cf7dp_datepicker_javascript', 'custom_date_picker_js', 10, 2 );

function custom_date_picker_js( $out, $field ) {
       // Target only the meeting date field.
       if ( $field->type != 'datepicker' || $field->input_name != 'date-553' ) {
            return $out;
    }

    $out .= ".datepicker('option', 'beforeShowDay', function (t) {
        var e = t.getDay();
        return [e == 3, \"\"]
    })";
    $out .= ".datepicker('refresh');";

    return $out;
}