Obtaining Blue Pikmin on Day 3 by using the scale glitch and going out of bounds, which allows for a 7 Day completion by completing Perplexing Pool in a single Day.
Be sure to watch with annotations turned on.
My theory on why this works:
Scale glitch: During cutscenes, scales will continue to move, but Olimar's position remains fixed. This lets you get inside of it if the scale is moving upward during a cutscene.
Warping to Blues: Normally, when you fall while out of bounds, the game warps you back to the ship; however, if you use the Napsack before you're warped, you instead are warped to a place nearby. I believe this is because the game treats you as if you're a treasure when using the Napsack, and treasures are warped close to where they fell.
This does not work in the Wii version. As you can whistle anywhere on-screen in the Wii version, the Blue Pikmin were changed to ignore your whistle until you have Yellow Pikmin, and they continue to ignore it even if you warp to them.
At the end of the video, I show a way to reach Bulblax Kingdom early using the scale glitch. This allows you to get everything in Awakening Wood in one Day except the Decorative Goo behind Blues; if someone can find a way to get that past the electric gate, a 6 Day completion should be possible.