When the Thinker says to the Flash "You should learn to lose every once in a while, Bar. A little humility will prepare you for what lies ahead", these are actually the Thinker's last dying words to Jay Garrick, the original Flash in the comics.
Cisco likens Harry to Ebenezer Scrooge and the Grinch. In reply, Harry claims that "you're a mean one, I wouldn't touch you with a 39 and a half foot pole", quoting the song "You're a Mean One, Mr. Grinch" from the 1966 TV special Dr. Seuss' How the Grinch Stole Christmas.
Cisco calls both Harry and Ralph humbugs, referencing Ebenezer Scrooge's catchphrase in the Charles Dickens novella A Christmas Carol. Cisco claims this as a "Festivus miracle", a phrase commonly used by Kramer in the TV series Seinfeld.
When Barry and Iris are opening presents from the wedding, there are 11 toasters in the scene.
Cisco calls Amunet an "evil Mary Poppins", referencing her English accent.