There's no better way to end the day than watching a beautiful sunset, and if you're lucky, you'll be treated to a unique phenomena. And you don't even need to travel to a far-flung locale to see it—it's visible from your own backyard.
When the skies are clear, look up (but not right at the sun) and just after sunset you could see it. What you ask? A quick green flash that happens just after the sun disappears from the sky. The optical illusion lasts for only a second or two, so you'll have to keep your eyes peeled.
While you can technically see the flash from anywhere, you'll have the best luck with an unobstructed horizon in front of you (think: wide open ocean or a vast field). It's also best to avoid areas where there's substantial pollution, or if there are clouds blocking the sun as it makes its descent.
What makes it possible for you to see the green flash? Refraction bends the sun's light. The atmosphere acts as a prism, separating the light into various colors. When the sun is fully visible, the different light rays overlap, so the individual colors can't be seen by the naked eye. But once the sun starts to disappear below the horizon, the colors of the spectrum disappear one at a time, beginning with those that have the longest wavelength.
The reason you see green specifically is because that's the color that gets through, and is seen more clearly. However, on especially still and clear days, you may be able to see touches of blue and violet too.
And the optical illusion doesn't just appear during a sunset—it can also be seen just before sunrise. So if you missed it the night before, you have another shot the next morning. But don't get too bummed out if you miss it again. You have two opportunities, every day, to view the phenomena.