JSFiddle - React, Tailwind, and code Playground

HTML

<div class="controls">
                                <fieldset id="Group1" name="Group1">
                                    <label><input id="Radio1" name="Radio1" type="radio" value="day" />Days of Week<br /></label>
                                    <label><input id="Radio1" name="Radio1" type="radio" value="month_day" />Days of Month<br /></label>
                                    <label><input id="Radio1" name="Radio1" type="radio" value="work_day" />Workday<br /></label>
                                </fieldset>
                            </div>
                                <fieldset id="Group2" name="Group2">
                                    <select class="form-control" id="days"  name="newday[]">
                                        <option value="monday">Monday</option>
                                        <option value="tuesday">Tuesday</option>
                                        <option value="wednesday">Wednesday</option>
                                        <option value="thursday">Thursday</option>
                                        <option value="friday">Friday</option>
                                        <option value="saturday">Saturday</option>
                                        <option value="sunday">Sunday</option>
                                        <option value="01">01</option>
                                        <option value="02">02</option>
                                        <option value="31">31</option>
                                        <option value="wd 1">WD 1</option>
                                        <option value="wd 2">WD 2</option>
                                        <option value="wd 25">WD 25</option>
                                    </select>
                                    </fieldset>

JavaScript

$(function() {
    // Group the select's options by the radio they correspond to
    var options = {},
        radios = $("#Group1 :radio");
    options[radios.eq(0).val()] = [];
    options[radios.eq(1).val()] = [];
    options[radios.eq(2).val()] = [];
    $("#Group2 select option").each(function(i, el) {
        var e = $(el);
        if (i < 7)
            options[radios.eq(0).val()].push(e);
        else if (i < 10) //38
            options[radios.eq(1).val()].push(e);
        else
            options[radios.eq(2).val()].push(e);
        e.remove();
    });
    
    // On radio's change, empty the select and append the options that correspond to the selected radio
    $("#Group1 :radio").on("change", function() {
        $("#Group2 select").empty();
        var arr = options[$(this).val()];
        for (var i = 0; i < arr.length; i++)
            $("#Group2 select").append(arr[i]);
    }).filter(":selected").trigger("change");
});