"Is 'The Love Song of Night and Day' a real poem? Is it printed anywhere?" the questions ask. And fine questions those are. After all, the title is attributed to the flavor text of seventeen cards in the Mirage and Visions expansions.
Yes, it is a real poem. Jenny Scott, a long-time editor at Wizards of the Coast and frequent contributor to The Duelist magazine, wrote it as part of the worldbuilding for Jamuraa, the part of Dominaria where the Mirage story takes place.
Here is the full poem with footnotes explaining which cards quote it.
Love Song of Night and Day
by Jenny Scott
He (Night) / She (Day)
Wrap yourself in your best bright clothes, your red and purple scarves of silk.
Run with me to the festival, where we will dance until sunrise.
The dwarves will beat their funny drums of zebra skins and hollowed trees,
while stiltwalkers perform, and the musician blows his bamboo flute.
And late in the night, the poets and storytellers entertain,
delight us with their dancing words, as we listen, clapping by the fire.
Enchant me with your tale-telling. Tell about Tree, Grass, River, and Wind.
Tell why Truth must fight with Falsehood, and why Truth will always win.1
I will tell my father's stories: how the giant mantis fooled Death
by holding still as a felled tree; how the elephants trampled
the leopard cub, and its father, though he knew, killed nine goats instead;2
how pirates gambled with a djinn and lost the thing more dear than gold.3
Tonight we'll eat a farewell feast. Cold corn porridge is not enough.
Let's peel papayas, pineapples, and mangoes, drink coconut milk,
and bake bananas.4 We'll dine on crocodiles, wild birds, and turtles,
perhaps a hippopotamus--if only you can catch it first.
I'll build a palace made of stone. Two hippo-headed guards will serve,
and tigers carry in your meals. I'll capture flying zebras
for your steeds, and fill the stable with every kind of unicorn.5
Butterflies and salamanders will decorate your garden.
I'll strand long strings of beads for you, blue, the color only kings may wear.
I'll carve a soapstone lioness, a wooden box to lock it in,
girded with sapphire amulets, ostrich feathers, ivory.
These things will protect you while I'm gone, remind you of my love for you.6
Your voice resounds like a songbird's, every word is a sweet, soft song.
When you run you're graceful and swift, sleek as a powerful panther.7
Mysterious chameleon, you're a thousand women at once,
sharp and strong as a lioness, yet gentle as a striped gazelle.
On this our last day together, let us walk across the grasslands.
Hold my hand and let's walk slowly, seeing everything as children.
Let's walk on the Daraja Plains, where leopards hang from trees, dosing,
tasseled tails swaying in the shade, near villages of tree-dwelling elves.
Glorious, to walk again across the savannah with my beloved.
A lion walks commandingly, a general among his troops,
camped the night before a battle. A snake, colorful and coiled, loops
around his bough, mischievous, hanging over the village path.
We'll find termites in their nests, hard tall towers above the plains,
and point-eared cats, taking their turns, guarding their many entrances.
We'll find the basket-nests of birds hanging from the acacia tree.
Rhinoceroses and dragons for once will let us walk in peace.
When lightning tears the sky's dark cloak and heaven's bird beats the water
on the muddy plains with its big wings, termites and frogs escape their homes
toward the lamps in the nearest village. Spiders dry themselves indoors,
the spotted lizards that never fall from ceilings suddenly appear.
In the forest, fires light the sky as the black clouds unfold their weight.8.
The black-and-white sacred monkey holds her children to her, and waits.9
Love, like lightning hits suddenly. It sparks the heart with blows of light,
its fire extending, bends, expands, beats and breaks your hiding places.
* * *
Remember when we were children, herding the sheep together,
leading them over the grassy hills with long sticks. Your silly songs
made me laugh, and in the evening, you'd enchant me with your stories,
lying on your back beside me. Even then my heart was yours.
I remember your sacred rites. You were so funny, so grown up,
so stiff and serious, all arms and elbows. You went in a girl,
but you returned a warrior. You marched back with the others--
your hair was cut, your eye tattooed with the red triangle of war.10
Tomorrow I must go, my love. I will tattoo my head with braids.
My shield will bear a shining sun so you will always be with me.
Inlaid with gold, it will shine like glowing embers.11 I will return
with lizard skins for your sandals. Paint your eyes black and wait for me.12
I am the sun, you are the moon. Wherever you lead I will go,
following across the wide sky, as long as I live and you love.
Sun follows Moon until she tires, then carries her until she's strong
and runs ahead of him again.13 I'll carry you, too, my beloved.
My love, we are not Sun and Moon. Instead we are like day and night.
The old ones say Day is a woman, who works only while it is light.
She herds her goats and catches fish, fills her fields with golden corn,
shows her children what is just and protects them from the cobra.
Day loves Night, who works in darkness, walking through heaven's milky sky
collecting stars with his quick arms, piling them into a basket
like a child collecting lizards and piling them into her pot
until the pot overflows with lizards, 'til the basket overflows with light.
Night wears a black cloak lined with fire, studded inside with gleaming stars.
At dawn and dusk he spies his love. Across the rolling hills of sky,
they glimpse each other--so briefly. They throw each other kisses, cry.
Their tears spill over Jamuraa. Mixed with blood, they wash everything red.14
But once, with a magician's help, Time was stopped and Day stood still.15
Night spread over Jamuraa, wrapped Day in his dark cloak and held her.
In their miraculous embrace, the two became as One. Until
pulled from Day's arms, Night sank, commanded by the western horizon that always beckons him to come.
I won't give up hope, my love.
Our love is like the river in the summer season of long rains:
For a little while it spilled its banks, flooding the crops in the fields.16
But soon it will evaporate with the dry heat. Like Day from Night,
I'll live my life apart from you, just glimpsing you across the sky,
because you cannot change, my dear, and nor can I.17
 "Enchant me…" - Village Elder, Mirage
 "I will tell my father's stories… how the elephants…" - Wild Elephant, Mirage
 "…pirates…" - Kukemssa Pirates, Mirage
 "Tonight…" - early harvest, Mirage and Sixth Edition
 "I'll capture…" - Zebra Unicorn, Mirage (note that "flying" was changed to "gentle" on the card.)
 "These things…" - Remedy, Visions and Sixth Edition
 "When you run…" - Panther Warriors, Visions
 "In the forest…" - Flare, Mirage
 "The black-and-white…" - simoon, Visions
 "…you returned a warrior… your hair was cut…" - Zhalfirin Knight, Mirage
 "My shield…" - blinding light, Mirage
 "I will return…" - Femeref Knight, Mirage
 "Sun follows…" - Chariot of the Sun, Mirage
 "Their tears…" - Mortal Wound, Visions
 "But once…" - Sands of Time, Visions
 "Our love…" - Summer Bloom, Visions
 "Like Day from Night…" - Unfulfilled Desires, Mirage