<label><input value="0" id="time_0" type="checkbox">0</label>
<label><input value="1" id="time_1" type="checkbox">1</label>
<label><input value="2" id="time_2" type="checkbox">2</label>
<label><input value="3" id="time_3" type="checkbox">3</label>

<input id="day" value="Пн" checked="" name="day" type="radio">Пн
<input id="day" value="Вт" name="day" type="radio">Вт
<input id="day" value="Ср" name="day" type="radio">Ср

$('input[name=day]').change(function() {
    $.ajax({
     dataType: "json",
     type: "POST",
     url: "time.php",
     data: {
            get: "",
            user: $('#user').val(),
            day: $('input[name=day]:checked').val(),
     }
     success: function(html){
         alert(html);
      }
   });
  });

$('input[name=day]').change(function() {
    $.ajax({
     dataType: "json",
     type: "POST",
     url: "time.php",
     data: {
            get: "",
            user: $('#user').val(),
            day: $('input[name=day]:checked').val(),
     },
     success: function(html){
         alert(html);
      }
   });
  });