Merry-xmas-happy-newyear.js

(function () {
  'use strict';
  var currentDate = new Date(),
      christmas = Date.parse('Dec 25, ' + currentDate.getFullYear()),
      newYear = Date.parse('Jan 1, ' + currentDate.getFullYear() + 1),
      daysToChristmas = Math.round((christmas-today) / (1000 * 60 * 60 * 24)),
      daysToNewYear = Math.round((newYear-today) / (1000 * 60 * 60 * 24));
  
  if (daysToChristmas === 0) {
    alert('Merry Christmas!');
  } else if (daysToNewYear === 0) {
    alert('Happy new year!');
  }
}());