@jonny But I suppose that doesn't actually address your original question of "when do we celebrate space's birthday?", because every coordinate transformation has some component of the old time in the new time. You can't treat a pure spatial direction as a time direction, unless you're really close to a black hole or something.
If the answer really is "monuments", then I might suggest the geodetic trigs? https://en.wikipedia.org/wiki/Triangulation_station