The official Splatoon Japan Twitter account posted a special tweet looking back at past events concerning Splatoon 2.
The first major date for Splatoon 2 is October 20, 2016, the day the game was first shown to the world. It was also the same day the Nintendo Switch was officially revealed to the world. Then on January 14, 2017, Splatoon 2 was officially announced at Nintendo Switch Presentation 2017.
— Splatoon（スプラトゥーン） (@SplatoonJP) June 24, 2019
A second part to this is expected to come in the next few days, as we approach the Final Fest.