My husband was naturalized in 2004 as a minor and his certificate doesn't say "effective date" but it has a date on it which was the date of naturalization. Are you absolutely sure that yours has no date anywhere on it? I find it extremely surprising that the certificate they gave you as proof of your citizenship is missing the date.
If you reapply for the certificate it's taking well over a year btw. You know you were a teenager at the time -any recollection of what grade you were in? What time of year it was? Just put your most accurate guess on there and write a letter of explanation and a copy of your certificate (iirc that is mandatory anyway for naturalized citizens).
I remember attending a small room in which you see a bunch of people who were to receive a certificate from a judge.
I was handed a piece of certificate and I thought that was it. The certificate tells you that its important to be an upstanding citizen and protect Canada etc. There is nothing on the back of the certificate. And I've kept it until this day. I believe that I received my citizenship card in mail. And "if" the citizenship card came with a formal certificate, I believe I would have kept it.
My parents received their citizenship cards in mail as well, and theirs have YY and MTH shown on their citizenship card. And mine, which was received in 99, shows YY-AP only. Quite strange.