Today I think is a good day for looking at the origin of day, as well as other times that have common names. Not midnight, though. I’m really hoping you understand that’s just mid- + night. And not evening, because I actually looked at it… holy crap, almost exactly eight years ago when I looked at even. Damn. I have been doing this a long time.
Anyway. Day, which I’ve at the very least mentioned before, comes from the Old English daeg, which in spite of having a G was actually pronounced “day”. Or really, more like “dahy”, but the point I’m trying to get across is there was no G sound there. That word comes from the Proto Germanic dages-, from the Proto Indo European agh-, which… come on, there’s not even a D there. And no one even knows where the D came from! It just showed up in Germanic at some point!
Dawn actually has the same Proto Indo European origin, just a slightly different path to get here. It showed up in the thirteenth century as dauen, from dauinge/dauing (which means dawn and is weirdly enough not where dawning comes from, as that’s just dawn + -ing). It’s from the Old English dagung, which means sunrise and as far as I can tell was pronounced “dayug”. That’s from dagian, another word for dawn and pronounced as it’s spelled, from the Proto Germanic dagaz, another word from agh-. So D’s just appear at random and G’s are just pronounced at random.
Now, morning showed up in the late fourteenth century, but it was actually a contraction of morwenynge/moregeninge, which showed up in the mid thirteenth century. You might think morn would be earlier, being the root word and all, but it showed up at the same time, the late fourteenth century, and again a contracted word, this from the Middle English morwen/morghen. It’s from the Old English margen/morgen (look, there were a lot of dialects and no fixed spellings back then), which is just morning. It’s from the Proto Germanic morgana, from the Proto Indo European merk-, which might be from the root mer-, to blink or twinkle. You don’t really see a lot of blinking or twinkling in the morning, though.
Noon showed up in the mid twelfth century as non, from the Old English non, which means the ninth hour after sunrise—okay, do you think sunrise is 3 a.m.? The reason actually requires a bit of explanation. Non is from the classical Latin nona hora, literally ninth hour, and the Romans had noon at what we would call 3 p.m., meaning sunrise was at 6. The shift from 3 p.m. to 12 p.m. happened during the twelfth century, although there’s no sure reason why. Theories include difficulty keeping track of time, especially up north, and people wanting to have their midday meals earlier. Seriously.
Dusk showed up relatively recently, in the early seventeenth century, although there did used to be an adjective form of the word—although weirdly, dusky was a word at the same time dusk would have been an adjective. Dusk comes from the Middle English dosc, meaning obscure or shadowy and referring more to color instead of lightness. It’s origin before that is actually unknown, as it doesn’t seem to be in Old English. There is however a particular dialect (Northumbrian) with the word dox, meaning dark-haired or dark from the absence of light. That does kind of make sense. Which is why we should be suspicious of it being accurate.
Finally, night. It comes from the Old English niht, night, from the Proto Germanic nahts. That’s from the Proto Indo European nekwt-, also just night, and the theory is that is from the root neg-, to be dark. As for why it has the gh in there, it’s because the gh used to be the letter yogh, Ʒ, which was like Y. People just started using gh in place of yogh, and then the letter disappeared altogether.
Tony Jebson’s page on the Origins of Old English