As to the issue of "every day" vs "everyday":
I tend to cherry pick my info
from general dictionaries and usage dictionaries and other sources, because, well, ...
So, ... To start off with, I'm a type of AmE speaker. (It is possible that the dialect of a speaker might be part of this issue; but I don't know.)
To me, the following example
might be usable to show a situation where the noun phrase "every day" has a meaning that is very similar to a meaning of the adjective "everyday":
1. In these jungles, a five minute shower of rain before breakfast is an every day occurrence.
2. In these jungles, a five minute shower of rain before breakfast is an everyday occurrence.
I could see #1 as maybe having two related interpretations: a) there is a morning rain burst each day; b) it is rather routine to have a morning rain burst and a person should probably expect to see one daily.
While for #2, I'd probably interpret as meaning that: a) it is rather routine to have a morning rain burst; b) it is not unexpected to have a morning rain burst; c) often there is a morning rain burst; d) almost
every day there is a morning rain burst.
Note that in #1, the noun phrase "every day" is being used as an attributive (or pre-head) modifier of the head noun "occurrence"--which is basically the same function that is being done by the adjective "everyday". That is, both phrases are fulfilling a similar function in the two examples: the function of being a pre-head modifier. I'm sorta thinking that it is because of that as the reason why the meanings of sentences #1 and #2 are so very similar, or could be very similar, to each other.
But for most types of sentences, I'm kinda expecting that the meanings of those sentences will be significantly different depending on whether they used the noun phrase "every day" or the adjective phrase "everyday".
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If we look at various sources--general purpose dictionaries and usage dictionaries--we might see various, er, definitions and explanations as to this issue of "every day" vs "everyday".