Celebrity's website says that they expect passports valid six months or longer. Whether they always enforce that rule, or monitor it for just when it applies I do not know. But if it were me, I would definitely get mine renewed if it were less than 6 months for that reason alone. Canada is technically not a country that requires the six months, nor does the US, but since the Celebrity website states it, then I would not want to take the risk. All cruises from the US stop in a foreign country (in this case Canada) due to some kind of maritime sailing rules about which I am no expert.
When renewing my son's passport recently, the turn-around time was just a couple of weeks. I guess it just all depends on the volume when you submit it. Would like to think it's our government improving processes, but the cynical part of me thinks a better explanation is that since the economy is still struggling fewer people can afford to travel so fewer are applying for passports. I am thankful that despite set-backs we can still fit a cruise or two into our budget (although we are more likely to downsize our cabin choice - sigh.......,)
While I am not recommending travel on passports with less than six months validity, I have used a passport with about five months validity on a Celebrity Med. cruise. It was fine for all of the Schengen countries, as well as Turkey.