How long before the passport expires can you apply for a marriage visa. I think I saw somewhere where it was mentioned 18 months. I was hoping to apply for a marriage visa about Christmas and if my pass port expires in April I would have to wait. Would appreciate some comment.

There is no specific marriage visa. There is a non immigrant visa valid for a single entry of 90 days which would require six month validity and a non immigrant multi entry valid for 90 day stays that is valid for one year and requires 18 month passport validity. So your passport would not qualify for either and you should obtain a new passport now (there is no need to wait until it expires and it is virtually useless now as almost all countries require six month validity for visa issue).