tagSprintWeekend2015.user.js

// ==UserScript==
// @name        SprintWeekend2015
// @namespace   tagSprintWeekend2015
// @match       https://www.drupal.org/node/*
// @run-at      document-end
// @grant       none
// ==/UserScript==

// SAVE as tagSprintWeekend2015.user.js
// Inspired by https://github.com/webflo/drupalsprint-bookmarklet/blob/gh-pages/index.html
// Modified from https://gist.github.com/clemens-tolboom/7e15dac5bfb07a729f6e.
// with the help of @helmo
function addJQuery(callback) {
  var script = document.createElement("script");
  script.setAttribute("src", "//ajax.googleapis.com/ajax/libs/jquery/1.4.4/jquery.min.js");
  script.addEventListener('load', function() {
    var script = document.createElement("script");
    script.textContent = "window.jQ=jQuery.noConflict(true);(" + callback.toString() + ")();";
    document.body.appendChild(script);
  }, false);
  document.body.appendChild(script);
}

// the guts of this userscript
function main() {
  var startdate = new Date('01/16/2015');
  var enddate = new Date('01/19/2015');
  var currentDate =  new Date();
  if (currentDate >= startdate && currentDate <= enddate) {
    // Uncomment if you want to limit the user script to a certain project on d.o.
    // if (jQuery('.field-name-field-project').find('.field-item').text() == 'Drupal core') {
      var tags = jQ('#edit-taxonomy-vocabulary-9-und').val();
  	  var add_comma = tags.length > 0;
  	  tags = jQ.map(tags.split(', '), jQuery.trim);
  
  	  if (jQ.inArray('SprintWeekend2015', tags) === -1) {
    		if (add_comma) {
    		  jQ('#edit-taxonomy-vocabulary-9-und').val(jQ('#edit-taxonomy-vocabulary-9-und').val() + ', SprintWeekend2015');
    		} else {
    		  jQ('#edit-taxonomy-vocabulary-9-und').val( 'SprintWeekend2015');
    		}
  	  }
    // } // IF END
  }
}

// load jQuery and execute the main function
addJQuery(main);