SOURCE

var startDate = document.getElementById('startDate');
//alert(startDate.value);
/*startDate.addEventListener("change",(event)=>{
  //alert("you changed the startDate");
  alert(event.target.value);
}, true);*/
startDate.onchange = (event)=>{
  alert(event.target.value);
}
var btn = document.getElementById('getData');
btn.onclick = (event)=>{
  var dataDiv = document.getElementById('weather');
  var request = new XMLHttpRequest();
  request.onreadystatechange = ()=>{
    //alert(request.readyState);
    if(request.readyState === 4 ){
      //alert(request.responseText);
      if(request.status === 200){
        //alert(request.status);
        var data = JSON.parse(request.responseText);
        dataDiv.innerHTML = '<h2>' + data.cityInfo.city + '</h2>' +
                            '<p>时间: ' + data.time + '</p>' +
                            '<p>当前温度: ' + data.data.wendu + '℃</p>' +
                            '<ol>明天(' + data.data.forecast[1].ymd + ')天气：'+
         '<li>'+data.data.forecast[1].high+'~'+data.data.forecast[1].low+'</li>'+
         '<li>'+data.data.forecast[1].fx+' '+data.data.forecast[1].fl+'</li>'+
    		 '<li>'+data.data.forecast[1].type + ' ' +data.data.forecast[1].notice+'</li></ol>';
      }
    }
  }
  request.open('GET', 'http://t.weather.sojson.com/api/weather/city/101210101');
  request.send();
  alert('request was send.')
}

<label><input type="radio" name="weekday" id="monday" value="1"> Monday</label>
<label><input type="radio" name="weekday" id="tuesday" value="2"> Tuesday</label>
<p><input type="date" id="startDate" value="2018-12-21"></p><button type="button" id="getData">Get Weather Data</button>
<div id="weather">
  
</div>
<div>
  <script src="http://www.weather.com.cn/data/sk/101210101.html" />
</div>