Pretty much everyone is allowed to defer an exam without a good reason, is only once through your whole degree though.
It happened to me last term, and I was able to write it during reading week, no complications at all.
Edit: I just realized you're in engineering and I actually saved the link for your form because I gave it to a friend a while back.
http://www.eng.mcmaster.ca/documents/deferred.pdf
just print this form, fill it in, and you should be fine.