Our Sun (Sol), and its system, takes a long time to orbit around the Milky Way Galaxy, ~225 million years, actually.
That’s too long for humans to find useful, so a group of (probably very smart) people decided to break it down into smaller chunks, about 1.736 years long (633.7 days), based on 1 centi-arcsecond of Sol’s orbit.
This period, of 1.736 years, is labelled a “Galactic Tick Day.”
It was retroactively decided that the first Tick Day would be October 2nd, 1608, though the first observance of a Tick Day was September 29, 2016. It’s a very recent phenomenon.
I decided to start this website to track these Tick Days.