It's a title with the word yesterday in it."All Our Yesterdays" should not be grouped with these others because it's a quote from Shakespeare (just like "By Any Other Name," "The Conscience of the King," and, of course, "Spock's Brain").

In one sense, Tomorrow is Yesterday and All Our Yesterdays belong together as time-travel eps, with Return to Tomorrow the odd one out.
Then it should be called "All Their Yesterdays."Yeah, but that's a quote from Macbeth - just like "Dagger of the Mind," and unlike the Tomorrow Episodes - to this day I have no idea what the title of Return to Tomorrow means in the context of that episode - it's a literary quote drawn from a near-universally-known passage (that was good enough for Faulkner) that also happens to describe perfectly what the inhabitants of Sarpeidon are doing: traveling back to all their yesterdays.
I agree with you on the vagueness of Return to Tomorrow but Tomorrow is Yesterday is as plain as day to me. They've traveled back in time; so what would have been a day in their 23rd century past (yesterday, poetically) is now a day in their 20th century future (tomorrow, poetically.)
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