Sinopsis
Follows directly after the events in Rules volume 3. This continues the tale of Hikaru, Yuki, & friends. A glimpse into Hikaru's life, past, and feelings. (Source: M-U)
Actually contains 2 parts, the title story, and a second story, Winter Bells, that takes place immediately after the first.
Complete information on the Rules series timeline can be found in the Synopsis for Rules.