Lovely Day is a story about a girl and guy having sex everyday over a seven day period. There really isn't much given to us other than they are alone together for this period of time and use this time to have sex in various ways.
The art is pretty good in the series, but the animation is really bad. The animation has awkward movement that reminds me of piece of paper moving to create motion/action. Its kind of weird to watch as the action isn't really clunky like you see in other lower budget hentai, its smooth but with no real movement outside of the piece of paper sliding around like action.
The sound and characters are both decent but neither are strong enough to really warrant much of a comment.
Overall the story and variety of sex keep this from being to bad, but the animation is to bad to really get into it. Which kind of leaves it in that mediocre range that no one really has much interest in. The only reason I'd suggest watching it would be to see the animation.