I scheduled a deposition of plaintiff for one day. How do I continue the following day if not completed on first day?

You should out in the deposition notice that it will be continued. You can send an amended deposition notice with that language. Or you could come to an agreement with the other side during the deposition. I am assuming you are in state court. If you are in federal court, this is not allowed. You are only entitled to one day at seven hours. Taking hundreds of depositions in non complex and complex cases, I would be surprised if you could not get your deposition done in one day, if it is properly focused.