Take a photo every hour throughout the day, and share on Instagram with the #1Day12Pics hashtag.
It's harder than it sounds. Some people use it as a photography creativity prompt (it's a good practise to get into, to have to look for photos wherever you are). Others use it to just share what they are doing for a whole day. Adventurous people pick a theme or a colour scheme, and try to find 12 photos in a day.
However you want to do it, it happens on the first Saturday of every month. If you don't want to fill up your Instagram feed, use the multiple uploads feature at the end of the day.