JSFiddle - React, Tailwind, and code Playground

HTML

<select id="Day">
    <option value="1">1</option>
    <option value="2">2</option>
    <option value="3">3</option>
    <option value="4">4</option>
</select>

JavaScript

var d = 3;
var daySel = document.getElementById("Day");
for(var i = 0; i < daySel.options.length; ++i) {
    if(daySel.options[i].value == d) {
       daySel.selectedIndex = i;
       break;
    }
}