This OVA happens at the same time of the events of the first episodes of Ten, up to the episode 4 (or 16), so I recommend seeing it after those episodes. Calling it 16.5 wouldn't be a bad idea.
Most of the important part that could be spoiler are from episodes 13 and 14, though, so you can watch it after those
It's not recommendable to see it before starting Ten, or just after the first episode, like the "13.5" in the mal name suggests.
About the story: it's a funny and a lighthearted story about Celty and Shinra's date in the mountain with some flashbacks with interesting info.
