There is a notable X-Men storyline from 1981 titled "Days of Future Past". It feels to me like it's a reference to some earlier piece of literature, but I cannot find anything in particular. There is a notable Moody Blues album from 1967 titled "Days of Future Passed", which I would have thought was a reference to the same thing, but which is the earliest thing I can find by either spelling.
Most references to the "past" spelling between the two dates seem to be misspellings of the Moody Blues' album, but I have found it as the title of an article from "New Times" magazine from 1978 having to do with Guantanamo. I find it hard to believe that this was a reference to a Moody Blues album, but maybe I'm wrong.
Since the early '80s, there seem to be a lot of references to "Days of Future Past" that aren't obviously related to either comics or prog rock. I find it hard to believe that this phrase was really popularized by a comic book in 1981, even one that was widely well accepted at the time, but I can't come up with any other conclusion.
There's even a poem from the TV show "Twin Peaks":
Through the darkness of future past
The magician longs to see
One chance out between two worlds:
Fire walk with me
Did this really get popularized by "X-Men"?