The event was constructed to help trainers complete a bunch of missions that would unlock the ability to capture Mew in-game. Some of the missions required trainers to catch 100 Pokemon in the Kanto Pokédex, evolve a Magikarp, and find a Ditto.
Over the course of the week, the spawn rate of the first 151 Pokémon was increased. The amount of candy trainers could get per capture was also doubled, making it easier to evolve all the generation one Pokémon.
Two of the game's rarest Pokemon, Snorlax and Aerodactyl, were also added to raids during this time, giving trainers a chance to battle and catch these beasts to add them to their Pokédexs. If you want to get these Pokemon before the event ends, grab a few buddies and head to your local gyms to hopefully find them waiting in a Raid.
Originally, dataminers discovered a bunch of new shiny Pokémon sprites in-game that many fans assumed would be part of the event. These shiny variants, however, have failed to appear during the event.
Niantic has given no indication if the event will return in the future, so fans should make the most of the increased spawn rate to get any remaining Pokemon that they need.