Who said: "The owl of Minerva takes its flight only when the shades of night are gathering," and what does this mean?

Only one word more concerning the desire to teach the world what it ought to be. For such a purpose philosophy at least always comes too late. Philosophy, as the thought of the world, does not appear until reality has completed its formative process, and made itself ready. History thus corroborates the teaching of the conception that only in the maturity of reality does the ideal appear as counterpart to the real, apprehends the real world in its substance, and shapes it into an intellectual kingdom. When philosophy paints its grey in grey, one form of life has become old, and by means of grey it cannot be rejuvenated, but only known. The owl of Minerva, takes its flight only when the shades of night are gathering.

http://www.autodidactproject.org/my/owl-tutorial.html

— Hegel's Philosophy of Right

In slightly more transparent language, philosophical ideas only describe the world as it is (or as it is becoming), they do not form it. 'The owl of Mionerva' is being used as a metaophor for intellectual understanding, and 'the shades of night are gathering' refers to the fact that events are already occurring.

No new reference, this is just an interpretation of the passage quoted above.

There are at least two points to consider.

(1) Hegel doesn't argue that philosophy ought not to preach to the world, but that it is not equipped to do so.

(2) This is because the fullness of comprehension of an epoch only comes into being as it reaches its end, and the new order that is coming into being has not yet penetrated consciousness to the point of being able to be articulated theoretically. If you think about it, this is what happens in the course of both an individual lifetime and in social development. Neither you nor the theoreticians of your society have a full conceptual grasp of what you lived through until the world you grew up in has changed so dramatically that the underlying assumptions of the time come into relief. But yet, as a product of an earlier time, you may not be fully in tune with what is going on now and cannot articulate its inner experience. This is what the generation coming up now will have to do when it gets an intellectual grasp on what it was like its inhabitants to live through this period.

http://www.autodidactproject.org/my/owl-tutorial.html

Maybe this says it even more simple.

Quote:
The nineteenth-century idealist philosopher G.W.F. Hegel famously noted that "the owl of Minerva spreads its wings only with the falling of the dusk" — meaning that philosophy comes to understand a historical condition just as it passes away. Philosophy cannot be prescriptive because it understands only in hindsight.

“One more word about giving instruction as to what the world ought to be. Philosophy in any case always comes on the scene too late to give it... When philosophy paints its gloomy picture then a form of life has grown old. It cannot be rejuvenated by the gloomy picture, but only understood. Only when the dusk starts to fall does the owl of Minerva spread its wings and fly."

https://en.wikipedia.org/wiki/Glaucus_%28owl%29

And here is a picture of the owl.

http://www.buildinghistory.org/bath/minerva.shtml