Turning night into day is a useful little trick that you'll need to use in Ocarina of Time. It's great to use when you don't want to have to mess around with battling Stalchilds in Hyrule Field either.
The Sun's Song is an Ocarina tune that turns night into day and day into night, and you can get it from the Graveyard in Kakariko Village. So, if you've just got your Hylian Shield for free and are still in the Graveyard, then you should do both at the same time!
If you're not in Kakariko Village, then head over to the right of Hyrule Castle Town, cross over the bridge, and head up the slope to the village. It's a lovely place filled with amazing characters and buildings, but the place you're heading to is the spooky Graveyard owned by Dampé the Gravekeeper.
Head right to the back of the graveyard to the large pillar. If you interact with the two gravestones on either side of the main pillar, you'll battle two Poes, the creators of the Sun's Song melody. If you defeat them, they'll apologise for attacking and explain how they had to write the melody for the Royal Family but died before they could finish it. But their secret is still hidden in their tomb.
Stand on the symbol of the Tri-Force and play Zelda's Lullaby. This will grant you access to the tomb below in the coolest way possible; a lightning bolt strikes the grave and decimates it, leaving a slope down into the tomb.
Head down into the tomb and through the first door you come across. There are 4 bats, or Keese in Zelda language, inside the first room that you'll need to get rid of in order to progress. Head up the stairs once they've been dealt with and carry on.
The next room has some weird green water and more terrifying ReDead zombies. You can either sneak past them, or run and hope that they don't catch you. If they do catch you, however, then you can just wiggle the control stick, and they'll let go. Although they're pretty good at squeezing the life out of you, and with only four heart containers at this point, they can get rid of your health pretty fast.
In the next room, Navi will spot some notes on a plaque. Get out your Ocarina and play the tune. You'll learn the Sun's Song, and you can use it to control day/night and also to calm the ReDed zombies. It puts them in a cocoon of light, and you can either use this to get past them unscathed or to keep them trapped while you attack them with your sword.
And that's that! You've learned the Sun's Song! Head out into Hyrule Field and start messing with people's body clocks!
