Early Sky Keep is a glitch using BiT that allows you to open Sky Keep on any file. This requires the use of a premade file (that already has the stone of trials).
The glitch requires two things to be setup ahead of time:
With this setup, activate BiT. You must now load the Stone of Trials into BiT. Obtain a Heart, Stamina Fruit, or Rupee while the file with the Stone of Trials is selected (or if that file is in slot 3, you should not select a file). Now go to the statue where you place the Stone of Trials and stand just out of its range. Select the file you wish to open Sky Keep on, press start, and after about 1 second place the Stone of Trials in the statue. This will cause you to Reverse BiTWarp into the Sky Keep opening cutscene.
If done correctly, the Sky Keep opening cutscene will play on the file you just loaded, and you will spawn next to the statue in Skyloft with all the other information from your file intact.
If the game crashes after this, then you placed the Stone of Trials too early after starting your file. Try to delay it a little more.
With this glitch, you can skip everything from opening the Gate of Time to entering Sky Keep, thereby bypassing all of the Song of the Hero quest and the Goddess's Silent Realm.
One caveat: while this glitch can be performed to open Sky Keep on a New Adventure or Hero Mode file, you will quickly dead end as Skyloft is set to a state where your loftwing cannot be rescued. Thus you will be stuck on Skyloft. If you wish to use this glitch, you should activate it from a save prompt after rescuing your loftwing.