Smells Like Teen Spirit - S6-E15
Trivia: Near the end of the episode, while dancing at the school dance, Beckett and Castle decide on Andrew Belle's "In My Veins" as 'their song' for the wedding. The same song plays under the end of "Always", the season 4 finale, when Beckett admits to Castle her feelings for him and they consummate their relationship.
Answer: The networks are now having winter breaks. A mid season end of new episodes, when they come back after the new year, it picks up as if several months have gone by. Beckett suggested Valentine's Day as a "chick flick" to watch as she is moved in. Also some episodes are not scheduled to air in order.