1. Today was Mid-Winter Day. It is a big holiday in Antarctica.

2. Today was Wnter Solstice Day. It is a big holiday in Antarctica.
3. Today was Winter Solstice. It is a big holiday in Antarctica.
4. Today was the winter solstice day. It is a big holiday in Antarctica.
5. Today was the winter solstice. It is a big holiday in Antarctica.
[Which expressons are grammatical?]

1 is correct

2 would be correct if Winter were spelled correctly.
3 is OK, but without the capitals W and S.
4 would be OK if capitalization were correct.
5 is correct.

Expressions 1, 3, and 5 are grammatical.

Explanation:

1. Today was Mid-Winter Day. It is a big holiday in Antarctica.
This sentence is grammatical. "Mid-Winter Day" refers to the middle of winter in Antarctica and is capitalized as a proper noun. The sentence is clear and makes sense.

2. Today was Wnter Solstice Day. It is a big holiday in Antarctica.
This expression is not grammatical. "Wnter" is misspelled, and "Solstice" should be capitalized as a proper noun. The correct spelling is "Winter Solstice Day."

3. Today was Winter Solstice. It is a big holiday in Antarctica.
This sentence is grammatical. "Winter Solstice" is correctly capitalized as a proper noun. The sentence is clear and makes sense.

4. Today was the winter solstice day. It is a big holiday in Antarctica.
This expression is not grammatical. "winter solstice day" should not be capitalized. Additionally, "Winter Solstice" is a proper noun and should be capitalized. The correct phrasing would be "Today was the Winter Solstice. It is a big holiday in Antarctica."

5. Today was the winter solstice. It is a big holiday in Antarctica.
This sentence is grammatical. "Winter Solstice" is a proper noun and should be capitalized. Since "winter solstice" is a common noun, it is not capitalized. The sentence is clear and makes sense.