dt = new Date();

m = dt.getMonth() + 1;
d = dt.getDate();

dtStr = m + "月" + d + "日";
link = "http://ja.wikipedia.org/wiki/" + encodeURI(dtStr);

document.write("<a href=\"" + link + "\" target=\"_blank\">");
document.write("今日は" + dtStr + "(Wikipedia)");
document.write("</a>");