365 Days is a Compilation Movie of Assassination Classroom, encompasing all the major events in the main series, as well as adding a new scene and dialogue for the compilation specifically, involving the 20 year old versions of Nagisa and Karma. Assassination Classroom was an amazing anime, and this abridged recap film pretty much does it justice.
Story: The story, as with any compilation movie, is very rushed to save time, and there are a few unexplained plotholes- but it mostly retains the polished shine the main series had despite these blemishes. It's a great story, and the narrative goes from silly and nonsensical to serious but lighthearted seamlessly. Everything is woven neatly together in a nice little package. However, If you want the fully fleshed out version of the story, it isn't this movie.
Animation: Even the nice extra scenes had great animation, and since they simply reused scenes and turned them into this compilation film, the quality is the same as the TV series, which is incredibly good. The art-style blends nicely with the flow and movements, and its very characteristic of the story. Lerche is a great studio.
Sound: No problems. The sound design was awesome, all the music is really good, as lerche OSTs seem to always be.
Characters: The series' strongest suit is cut completely in half for this compilation movie. The strongest character's strongest traits remain, but with so many side characters converted into background characters, it's hard to get excited. Nagisa is the outstanding character, of course, with an incredible character progression i've barely seen anywhere else. Koro-sensei is loveable and awesome as ever! Even cut short he shines through as the core of the anime. Karma is ok.
The ending was great, even though the movie doesn't have quite as much buildup to the finale as it could have. I would have preferred that they focus a bit more on the dramatic moments leading up to it that make it so much more impactful, maybe sacrificing a few sidestories to do this. Overall, 8/10. Pretty good movie. Not the full experience if you're trying to watch it to start the series with. Best to watch it afterwords.