In my post yesterdays' post, I wrote, "3 Days Trip" as the title.

Then guys corrected me likey title to “A Three Day Trip.”

I made senseunderstood “A Trip” but I didn’t make senseunderstand the reason why it was “Three Day” instead of “Three Days.”

Make sense = passive verb = "me". "A Trip" made sense TO ME
understand = active verb = "I". I understood "A Trip"

For a song's’ title, does “SEVEN DAYS WAR” does it sound OK?

Or should they have titled it “A SEVEN DAY WAR” ?

そうです！ "Our Seven-Day War"

By the way, the movie was popular foras a refreshing youth films.

In my yesterdays' post, I wrote, "3 Days Trip" as the title.

Then guyspeople corrected me likewith “A Three Day Trip.”

I made sense out of “A Trip” but I didn’t make senseunderstand the reason why it was “Three Day” instead of “Three Days.”

It was about a song's’ title called “SEVEN DAYS WAR” by TM NETWORK.

The song was for the movie called “ぼくらの七日間戦争( (OUR SEVEN DAYS WAR).”

For a song's’ title, is “SEVEN DAYS WAR” does it OKokay?

It sounds funny to me but it's their song so they can name it whatever they want.

Or should they have titled it “A SEVEN DAY WAR” ?

Yes, I think so but this is just my preference.

By the way, the movie was popular for refreshing youth filmsviving films about youth.

In my yesterdays' post, I wrote, "3 Days Trip" as the title.

you could also have written "in the post i made yesterday"

I “A Trip” made sense “A Trip” but I didn’t make senseunderstand the reason why it was “Three Day” instead of “Three Days.”

For songs’ title, “SEVEN DAYS WAR” does it OKis that okay?

Or should they have titled it “A SEVEN DAY WAR” ?