Where is "the" used? Is there a rule? Why no definite article in "Shops are open late in summer"?

Notice these sentences:

1. Shops are open late in summer.


2. Summer is a traditionally viewed as a slow season for games.


3. At the summer solstice, the days are longest and the nights are shortest.


4. it's time to start thinking about what 2016's Song of the Summer will be.

In English class, they told us that "the" is used for the things that are known. One example for this was a phrase like "The man who killed his wife". So when we are speaking of that man, we know what he did. Somehow we know something more than nothing about him. He's not just every man in the world.

So Now I'm confused about why "the" is not used in the first and second sentences. Can anyone please explain it for me?

Answer

"Summer", like the other season names, is often treated as a name (and sometimes written with a capital letter, like other names). Examples 1 and 2 do this.

Example 3 is not a counter-example: "summer" is there used as a modifier for "solstice", which is a common noun and requires an article.

Example 4 is a bit more complicated. "Song of Summer" would be possible, but would suggest Summers in general; "Song of the Summer", especially since a year is given, suggests that particular Summer.