Sekai Days (Also written as セカイデイズ) is a trailer for a supposed upcoming game or anime by 0verflow released in April 1st of 2008, centering apparently on Sekai Saonji. The contents of the trailer don't have any relevance, though, since it's only an April Fools' joke.
The video begins with a parody of the Metro Goldwyn Mayer intro, only with Sekai roaring in place of the lion. After that, it fades to black into a bit of text telling about how ``It is an event of a certain rain on the day.´´, followed by the actual trailer.
Sekai is shown sitting in a restaurant and wearing her school uniform, outside the clouds are covering the sky but the weather is calm. She is happily looking at a photo of Makoto Itou on her phone as she reminisces about him, with the implication that she is waiting for him. However, times passes on, shown by the weather turning into a heavy rain and a shot of the clock in the restaurant.
Customers come and go silently while Sekai keeps waiting, until finally she cries, gets up from her chair and grabs her briefcase. The screen fades to white and the last thing heard is the sound of the restaurant's door opening and the falling rain.
Another line of text tells that ``The story starts here.´´ and the Sekai Days logo is shown as Sekai reads the name. A second later, it cuts to three images of Taisuke Sawanaga shirtless (the third one showing him kissing with Makoto Itou) before announcing Taisuke Days (たいすけデイズ), with the title read by Sekai too. This is of course another joke.
The last thing shown is the address for 0verflow's webpage.
https://www.youtube.com/watch?v=zL29qvSJr-s The trailer on Youtube
http://nviewer.mobi/player?video_id=sm2869458 The trailer on Niconico