The feature works much like the Timehop posts you've no doubt seen on your Facebook news feed, surfacing things that you either posted or were tagged in from a specific date in the past. From Facebook:
After checking out "On This Day" for the first time, users also have the option to subscribe to receive notifications whenever there is something to check out.
Facebook notes that users will be able to access "On This Day" on the web, as well as through a menu item in its mobile apps. The feature is still rolling out to users, however, and doesn't look to be live as of this writing.
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