Nietzsche’s most famous articulation of eternal recurrence of the same is imagined as a thought experiment. The question Nietzsche poses is, ‘Would you live this life over again under the same conditions?’
After reading the quote, think of Bill Murray in Groundhog Day and the allegory makes more sense.
The greatest weight.-- What, if some day or night a demon were to steal after you into your loneliest loneliness and say to you: "This life as you now live it and have lived it, you will have to live once more and innumerable times more; and there will be nothing new in it, but every pain and every joy and every thought and sigh and everything unutterably small or great in your life will have to return to you, all in the same succession and sequence - even this spider and this moonlight between the trees, and even this moment and I myself. The eternal hourglass of existence is turned upside down again and again, and you with it, speck of dust!"
Would you not throw yourself down and gnash your teeth and curse the demon who spoke thus?... Or how well disposed would you have to become to yourself and to life to crave nothing more fervently than this ultimate eternal confirmation and seal?
- Friedrich Nietzsche
The Gay Science, s.341
translated by Walter Kaufmann