Most of "Not A Father's Day" is about changes that the gang is going through — just not right now. Robin is between houses, living on Lily and Marshall's couch. Lily and Marshall are thinking about having kids. And Ted is still trying to recover from near-matrimony and find his comfort zone back as a single person. Things are in transition, but as symbolized by this episode's Barney coinage — the Cheerleader Effect (girls who seem more attractive as a group than than they are individually) — things are really just the same.
Should Lily and Marshall have kids? Lily drains a bottle of red and lets Robin and Ted, representing con and pro respectively, make their arguments. But their mixture of logic and ad hominem attacks has no effect on baby-crazy Lily. All drawbacks of parenthood are trumped by Sock. Sock, sock, sock! Baby sock, so tiny, so cute! The way Sock miniaturizes normal adult footwear is irrefutable. And as a parent, let me confirm that this is absolutely true. Baby things overwhelm rational though. Sock! Even Robin can't resist its scaled-down-edness. By the end of the episode, she can't deny its siren song. Sock!
Barney, on the other hand, is thrilled to get a phone call confirming that he isn't a father. In a brief, hopeful moment for us fans of his Broadway work, he dances down the street high-fiving random New Yorkers, but unfortunately not bursting into song … yet. (Surely the musical episode of HIMYM is still to come.) Barney invents Not A Father's Day, complete with real (of course) website notafathersday.com and associated merchandise. (The academic geek in me appreciates the footnote for Notable Not-Father Immanuel Kant — "pure reason says no kids" — but deplores the lack of any mention of the Latin motto of Not A Father's Day, visible in part on the banner in the last bar scene.)
This isn't a transcendent episode, but you've got to appreciate the good moments for all members of the ensemble (Robin riffing on Marshall's "1950's space helmeet of a head," Marshall sadly telling Lily that "it's breakfast time in China," Barney gulping after his cold-open phone call, Ted being "not mad, just disappointed" when Robin and Barney break his ship-in-a-bottle). There's plenty of inventive comedy here grouped around one of the oldest gang-of-friends-hanging-out plotlines in the book (the consequences of coupleism and the biological clock). I especially like Robin interrupting the baby-making session on the couch in order to watch her TiVoed The View (is she pouring beer into a tub of ice cream, though?), Robin's reaction to Lily's sudden confession that "dinner is a baby!", Robin letting the baby sniff her, Robin saying "sock!" … Hm, for an episode that didn't seem to be focused on Robin, she got a diaper-load of good moments.