notfromhere; I think this paragraph from the original post may answer your question;

Grand Mufti Abdul Aziz Al Shaikh said celebrating such events would make Muslims like followers of other faiths including Jews and Christians

So it looks like it isn't forbidden by the Quran, but because other religions do it Muslims shouldn't (according to some people).

So that raises the question; why is it considered bad for Islam to adopt things from other religions?