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

Question

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?]

Answer 1

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.