There is a certain hour when today, tomorrow and yesterday happen simultaneously on Earth. Since there is a stretch of time zones to 27 total hours, this makes it possible for three days to exist all at once on our planet. Although locally within your time zone it is one day or the other, it is upon the global perspective that all three happen on planet Earth.
It happens from 10am to 12pm London Time, aka GMT or UTC. Consequently this flag can be flashed or flown during that window: when today, tomorrow and yesterday exist on our planet. For the East Coast of the USA, this happens during standard time from 5am to 7am. But for the West Coast of the US, this happens from 2am to 4am. While in Japan and Korea this takes place 7pm to 9pm-during standard time. Since it happens simultaneously, it will vary for whichever time zone one lives.
The Today, Tomorrow, Yesterday Flag is a vertical tribar of orange, yellow and purple. The repeat of the Roman Numeral III indicates this happens for two hours. And the numeral three represents today, tomorrow, and yesterday. Also, the yellow bar represents today, the orange represents tomorrow, and the purple represents yesterday.
But for the people of Kiribati it can also be called the hour of today, yesterday, and the day before yesterday. And those in the last two time zones it can be known as the hour of today, tomorrow, and the day after tomorrow.