Not long after you arrive in Akala Island, you will be given the Zygarde Cube key item, which allows you to collect the numerous Zygarde Cells and Zygarde Cores scattered across Alola. In total, there are 95 Cells and 5 Cores, so 100 in all.
Cells and Cores can be found literally anywhere, outside on the field or inside buildings. They are even found on Melemele Island, before you were given the Zygarde Cube, but only appear after you've gotten the Zygarde Cube. Needless to say, you've got your work cut out for you.
That said, usually they aren't too difficult to find if you keep your eyes peeled and look in every nook and cranny. The kicker is that some Zygarde Cells only appear during the day or the night. So you may need to visit some locations twice to collect 'em all. Oh boy...
If the time of day is about to transition from day to night or vice versa, beware that the Zygarde Cells that only appear during the day or night only change after you enter a location.
For example, if you wait in a particular location until it changes to night, the Zygarde Cells that appears at night won't appear until you leave and come back. Nor will the day-time Zygarde Cells disappear.
As if this wasn't already becoming one of the most tedious collectathons ever, the game only bothers to track the total number of Zygarde Cells and Cores you've collected, which you can check by using the Zygarde Cube item from your bag.
So it won't give you any information about the areas where you've discovered all the Cells or Cores or anything remotely helpful like that. Fortunately, we'll be listing all of the locations over on the next few pages, so you can check to see which ones you've got or still need.