tomorrow: the day right after the one now in view, and by extension the time ahead that nobody yet holds
Greek names the day after the present one with a single small adverb, and it does not swap that word when the vantage point moves. Spoken from where a person is standing, it is "tomorrow": the plotters in Acts 23:20 want Paul brought down to them tomorrow. Told from inside an account that has already reached a particular day, the very same word means "the next day": the Samaritan pays the innkeeper the next day in Luke 10:35, and the arrested apostles are held until the next day in Acts 4:3. Greek does have a dedicated compound for that storytelling move (ἐπαύριον), but the plain word does the same job once it takes an article.
With an article in front of it, this word behaves like a noun, something close to "the morrow." A day a person can talk about, rather than simply act in, can also be handed a life of its own: Matthew 6:34 says the morrow will do its own worrying. From there it is a short step to standing for the whole stretch of time nobody has been given yet, which is what James means when he tells confident traders they do not know the first thing about tomorrow.
The word travels almost everywhere with its opposite, "today" (σήμερον), and the pairing is where its force comes from:
- Grass that is alive today is oven fuel tomorrow, in Matthew 6:30 and Luke 12:28.
- Jesus drives out demons today and tomorrow, and on the third day he finishes, in Luke 13:32.
- Merchants announce that today or tomorrow they will set out for a city and trade there for a year, in James 4:13.
Each pairing draws a very short line, and the writers use that shortness to say how little of it belongs to us.