Now in the past

I've hear some English native speaker using in the past adverbs like now, tomorrow, today and others I can't remember now instead of what it seems more suitable for me, maybe just because of my native tongue, like then, the next day, that day.

Is that a normal practise?

Just in case you need an example: I was seeing the doctor tomorrow (talking about last week)

I don't know. I don't think I would say something like that. I would probably say "the next day" if I was talking about a day last week.

On the other hand, I might say that sentence in a context like "I was seeing the doctor tomorrow, but I changed my mind." where I'm talking about a plan that I used to have for tomorrow, but even then it sounds kind of funny to me. I would probably say "I was going to see the doctor tomorrow, but I changed my mind." instead.

"now" in the past would be "then", no ?