Natsu no Daisankakkei was created on 2012-05-16.
Natsu Koi Natsu Game was created on 2009-08-12.
Chirusoku no natsu was created in 2003.
Natsu e no Tobira was created on 1981-03-20.
Natsu ni Koisuru Onnatachi was created on 1983-08-05.
Kimi ga Ita Natsu was created on 1992-08-21.
Natsu no Owari ga Mada Koko ni Aru was created on 2007-07-25.
Natsu no Yuu-utsu - Time to Say Good-bye - was created on 1995-10-20.
END stands for Etherious Natsu Dragneel, Natsu is Zeref's younger brother, who died young. Zeref seems to have used his brother's body to create an ultimate demon called END. Natsu was then given to the dragon Igneel to raise, and then sent 400 years into the future through the original Eclipse Gate. The demon part of Natsu is sealed within the book of END, he is tied to the book somewhat, and to Zeref. Damaging the book injures Natsu, and Zeref said that if he dies, Natsu will die too. In the end, Natsu denied both the dragon and the demon sides of him, and Lucy rewrote the book to prevent his death when Zeref died.
Natsu means Summer in Japanese.
The fire dragon Igneel raised Natsu.
The duration of Natsu e no Tobira is 3600.0 seconds.
Natsu, probably.