"Tomorrow will be December the 20th" should be accepted. It's standard in Australian English, though we're more likely to say the number before the month.
Hell in some regions people say "twentieth December" (with an implicit "day of" in between). I almost feel more tested on my English than on Japanese here!
Is "December THE twentieth" really wrong?
No. It's just the answers are for American English. The answer should be correct
Maybe it's a regional thing but I could see it being used either way where I live (in America).
Hey, that's my birthday!
why is the 二十日pronounced hatsuka?はつか
That's just the way it is. The number twenty seems to be important for the Japanese. For example: "twenty years old" in Japanese is: 二十歳（はたち）. All other ages end in -さい: 一歳（いっさい）、二歳（にさい）、三歳（さんさい） etc.
"Why are there special readings for certain days of the Japanese Calendar? These “special readings” are simply remnants of the original counting system from before the Chinese counting system was introduced."
Is it correct to say that in English using the present tense? "Tomorrow is December the 20th"?
Yes, that's correct.
Why is 明日は十二月二十日です Wrong?
明日は十二月二十日です Is not considered correct
Yeah, what? I got this wrong, too, and it makes no sense.