Princess Celestia rewards Twilight Sparkle with a talisman called the Chronex. With it, Twilight has the ability to visit any moment of our human history as it happened. With this, she can also bring her friends along to experience the same. In turn, Celestia designates each of the Mane 6 a different aspect to study:

Rainbow Dash would explore our world's military history, such as the Battle of Thermopylae, the Iraq War, etc.

Fluttershy would explore our world's natural history, such as the extinction of the Dodo bird, the Mount St. Helens eruption, etc.

Rarity would explore our societal history, such as the Code of Hammurabi, the Civil Rights movements, etc.

Pinkie Pie would explore our cultural and entertainment history, such as the painting of the Sistine Chapel, Woodstock, etc.

Applejack would explore our business and economic history, such as Marco Polo's trade route, the subprime mortgage crisis, etc.

Twilight Sparkle would explore our scientific and technological history, such as Newton discovering gravity, the invention of electronics, etc.

That's not to say only one at a time. All of the Mane 6 can be in different periods at different locations (using the Elements of Harmony to keep in touch), or studying a related subject (the invention and life of video games would be something Twilight and Pinkie would check out).

NOTICE:
This fanfic and its subsequent stories may contain sensitive content and suggestive situations to some readers which are integral to the narration, as they reflect on actual events in history. It is appropriate for those who can read the stories bearing this in mind. A mature audience is suggested.

The stories in this fanfic tell various events of history in a fictional and looser nature, to avoid over-detailing discrepancies, preventing accidental/unintended biases, and to maintain a shorter, fictional, cartoon-like feeling intended for the stories. Any other minor form of actual happenings are purely coincidental.

None of the stories reflect the views of the writer, readers, et al, in any way whatsoever. This work is purely for entertainment purposes.