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The Sun's Song is an optional but very useful song in Zelda: Ocarina of Time.
Allowing you to change from day to night (or night to day, depending on the current hour), it means you can quickly trigger unlock requirements for various side-quests, source Gold Skulltulas who come out when the sun goes down, and open shops if you happen to visit too late.
The good news is you can unlock Sun's Song fairly early in Zelda: Ocarina of Time - here's where to go.
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How To Get Sun's Song In Zelda: Ocarina of Time
First, you'll need to have unlocked Zelda's Lullaby. You'll get this by sneaking through Hyrule Castle and meeting Zelda for the first time, which is a mandatory story quest once you leave Kokiri Forest at the start of the game.
From there, to get Sun's Song, head to the Kakariko Village Graveyard. This is found in the south-east of the village, where you'll come to a dark and rainy network of pavements and gravestones.
You want to head right to the very back of the Graveyard, where there's a large tombstone with a Triforce on the ground in front.
Play Zelda's Lullaby in front of it on your ocarina, and the tombstone will slide open and reveal a mini-dungeon inside.
And we do mean mini - this will only take you a few minutes to complete.
Royal Family's Tomb
In the opening room, defeat the Keese with the Slingshot to open the way forward.
Climb up the steps and head through to a room filled with ReDeads (zombie-like creatures), where the best way to avoid them is to run through the middle of the room, poison water and all.
In the end room, interact with the tablet and learn the Sun’s Song on the ocarina, allowing you to turn day into night, and vice versa.
Now head back out into the Graveyard. A little tip on the way out: playing Sun's Song near these ReDeads (and their mummified cousins, the Gibdo) will stun them in place, allowing you to get close and finish them off with a sword. It's worth remembering this when you encounter them in later dungeons.
When you're ready to continue your adventure, assuming you've come here as soon as you are able, show Zelda's Letter to the guard in the north exit, then make your way to Goron City and ultimately Dodongo Cavern. Otherwise, if you're in the mood for another side-quest, then the Happy Mask Shop quest awaits.