The riddle
What Always Comes but Never Arrives?
One day ahead
Tomorrow is the day after today. The moment midnight passes, the tomorrow that everyone was waiting for becomes today, and a new tomorrow stands one day further on. Tomorrow is always coming and never here, because nobody ever lives in it. The word "arrives" is where the logic bites: whatever arrives has already stopped being tomorrow. A chore put off until tomorrow meets the same fate: when tomorrow comes, it is today, and the chore is still waiting.
Guesses that almost work
The weekend and a birthday both come around, so they make natural first guesses. They fail on the second half of the clue, because they really do arrive. Listeners who stay with "always" and "never" usually reach tomorrow.
Where the word comes from
"Tomorrow" goes back to Old English "to morgenne", which meant "on the morrow". It joins "to" with a form of "morgen", the word for morning. "Morrow" is the same word without its first part, and it survives in old-fashioned phrases such as "good morrow".
A relative in 1915
Edith B. Ordway included a cousin in her 1915 Handbook of Conundrums: "What is that which will be yesterday, and was to-morrow?" The printed answer is "To-day." The hyphens are the spelling of the time. Put next to the modern riddle, it shows the same idea from the other side: today is the tomorrow that finally arrived.