Source code for https://formidable-masterminds.com/display-day-name-on-date-field-change/

display_day_name.js

<script>
jQuery(document).ready(function($) {
    "use strict";
    
    /* change field_s32q2 to the id of your date input field
     * use your browser's inspection tool to verify the id spelling
     */
    $('#field_s32q2').on('change', function() {
        /* create a constant reference to a day name values */
        const days = ["Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"];
        
        /* create a working date variable from the value of the input field */
        var selected_date = new Date($(this).val());
        
        /* display the day name in your text or hidden field
         * change field_qh4icy2 to the id of your text or hidden field
         * use your browser's inspection tool to verify the id spelling
         */
        $('#field_qh4icy2').val( days[selected_date.getDay()] );
    });
});
</script>