1. Today was Mid-Winter Day.

2. Today was Mid-Summer Day.
3. Today was Mid Summer's Day.
4. Today was Mid Summer Day.
[Which expressions do we use?]

Could use any.

To determine which expressions to use, we need to understand the correct grammar and punctuation rules for referring to specific days or events.

1. "Mid-Winter Day" - This expression is correct if you are referring to a specific day in the middle of the winter season. The hyphen (-) is used to connect the words "mid" and "winter" to create a compound adjective.

2. "Mid-Summer Day" - This expression is correct if you are referring to a specific day in the middle of the summer season. Similar to the previous example, the hyphen (-) is used to connect the words "mid" and "summer" to form a compound adjective.

3. "Today was Mid Summer's Day" - This expression is incorrect. When indicating possession (in this case, the possession of the day by summer), the correct form is "Midsummer Day" without an apostrophe.

4. "Today was Mid Summer Day" - This expression is incorrect. It should be written as "Midsummer Day" with "mid" and "summer" joined by a hyphen to form a compound adjective.

So, the correct expressions would be:
1. Mid-Winter Day.
2. Mid-Summer Day.
3. Midsummer Day.

When unsure about the correct usage, it is always recommended to consult reliable reference sources or style guides to ensure accuracy.