I'm not a linguist, so take everything I say with a grain of salt, but I'd suggest that:
By adding the は here (which is an unusual thing to do), you're treating 毎日 as a topic and a noun.
Which makes it: As for every day, I go to the park.
Instead of: I go to the park every day. (where "every day" 毎日 is an adverb)
The meanings are close but slightly different. Given the question, I'd probably consider it to be wrong.
But "As for every day, I go to the park" its the same meaning right? But an adverb is where it describes about the verb...so how is every day an adverb?
I remember finding it kind of interesting that Japanese and English are almost identical in how they do time expressions. What I mean by that is, Japanese only seems to use the particle に if we would use the word "in", "at", "on", etc. in the same sentence written in English. I might be wrong on this, there could be some cases that are different (if there are, please tell me, I'm curious to know too!), but generally that seems to be the case and I think that's such a coincidence, take a look⬇️
EverydayI go to school ⇔ 毎日学校に行きます (You wouldn't say "in/on/at everyday")
I will go to work on Monday ⇔ 月曜日に仕事へ行きます (Ok fine you caught me, this one I'd probably just say "I have work Monday" but who knows if that's even grammatically correct English lol)
YesterdayI cleaned the house ⇔ 昨日、家を掃除しました。(You wouldn't say "on/at/in yesterday")
I plan to go on vacation in July ⇔ ７月に休みの予定です。(You wouldn't say "I plan to go on vacation July")
Hopefully this helps as a mnemonic at least a little
But "As for every day, I go to the park" its the same meaning right? But an adverb is where it describes about the verb...so how is every day an adverb?
Again, I'm not a linguist, but I would suggest that, in the sentence "I go to the park everyday":
"every day" is an adverb (technically an adverbial phrase) describing how often we are "going". How often are we going? every day
If you replace "every day" with the adverb "frequently" it should be a bit clearer what is going on.
"I go to the park frequently" sounds good, but "As for frequently, I go to the park" sounds very weird.
You could construct sentences with 毎日は where you use "every day" as a noun (where you are talking about what "every day" is, e.g. "Every day is an adventure", or what "every day" is doing), or sentences where you are contrasting the frequency of "every day" with some other frequency, but neither of these are happening in the original sentence.
