Watch 'Act your age' Season 3. It explains it all.
May/December is an older man going out with a much younger woman (like the POTW's dad, who was secretly dating their playgroup teacher.)
Cuddy made a comment to House on the balcony-- CUDDY: I don't get the whole May - December thing.
HOUSE: Give me a break, it's May - October at worst.
CUDDY: Well why can't it be October - October?
HOUSE: May is when things start to get hot.
CUDDY: [Laughs.] If that guy wasn't trying so hard to keep up with a women half his age he wouldn't have almost killed both his kids.
HOUSE: Guy gets a little something-something, couple of kids have to die, circle of life.
CUDDY: So many people, so much energy and drama just trying to find someone who's almost never the right person anyway. It just... shouldn't be so hard.
It refers to the character's ages. May - December being Cameron (late 20s early 30s) and House (40s), and therefore House and Cuddy are similar ages October-October (40s)
A May-December relationship is one where one of the individuals is significantly older than his or her partner. October-October comes from a discussion between House and Cuddy. A man had basically poisoned his two children because he was trying to live up to his younger girlfriend's "expectations." When Cuddy questioned the whole May-December romance, House told her that for that couple it was more like "May-October," implying that the man was middle-aged instead of being much older than his twenty-something girlfriend. Cuddy's response of "Why can't it be October-October?" refers to two middle-aged individuals starting a relationship (aka House and Cuddy). This scene takes place in the season 3 episode "Act Your Age"
