SO MANY SPOILERS. These are in vaguely chronological order in the episode. Some more significant than others. I'll probably add to this once Tom & Lorenzo's fashion recap comes out.
  1. Fur: Don started working in advertising by doing ads in-house at a fur company. It's how he met Betty (she was a model) and how he met Roger (he came in for a fur for Joan, as seen in season 4, episode 6: Waldorf Stories).
  2. The diner dinner: a twist on the dinner in Ladies Room (season 1, episode 2) with Don, Betty, Roger and Mona. In season 1, Don refuses to talk about himself. In season 7, 10 years later, Don's telling Dick Whitman stories.
  3. Roger's roach joke in the diner calls back to The Suitcase (season 4, episode 7). Peggy asks, "Why is there a dog in the Parthenon?" Don says, "That is a roach."
    Then Don says, "Let's go someplace darker."
  4. "Do I know you?" Don says this to the diner waitress and also in The Crash (season 6, episode 8) to Ted's secretary Moira right after he gets his "proprietary" vitamin injection.
    This kicks off his quest to find the soup ad, intercut with the flashback to the boarding house with Aimee, and we see how Sylvia looks like the woman in the ad, who looks like Aimee.
  5. Tricia the flight attendant was in the first half of season 7--episode 3 (Field Trip) where Don goes to surprise Megan in LA.
    She says "I hate your wife."
  6. The wine spill on the carpet is reminiscent of the post-party cleanup in A Little Kiss (season 5, episodes 1&2), and of Bobby filling Emile's fountain pen later that season in At The Codfish Ball (episode 7).
  7. Also: the location of the wine spill is where Don strangled Andrea in his fever dream in Mystery Date (season 5, episode 4).
  8. The earring Tricia finds under the bed is one we saw Megan wear a few times in Season 6, but never in Season 7. Guess it's been lost for a while.
  9. "You want a raise, stop acting like a secretary." Peggy says this to Mathis, and Don said it to Peggy in Season 3, I think? Still trying to confirm this one.
  10. Rachel Menken was in the first episode of the first season, and now the first episode of the last season. (Half-season, whatever. It feels like a new season.)
  11. A woman showing up unexpectedly to the office wearing fur is reminiscent of Pete's fantasy about Beth Dawes in Dark Shadows (season 5, episode 9).
  12. There's a poster behind Meredith's head of an Accutron ad. This was the opening of the 1st episode of the last half-season.
  13. Don is visited by a woman he loved just before finding out she died, like with Anna in The Suitcase.
  14. Joan and Peggy's meeting with McCann is like a remix of any meeting in Season 1: just imagine it without the women there.
  15. Joan and Peggy's elevator conversation has echoes of the one they had in The Summer Man (season 4, episode 8) when Peggy fires Joey for being disrespectful and they butt heads about their different styles of dealing with that behavior.
    Also, Joan spitting "I don't dress the way you do because I don't look like you" is a bitter twist on Peggy's rejection of Joan's advice in season 1.
  16. "It wasn't meant to be." Peggy says this to Mathis when it looks like the date won't work out. It was said to her by the real estate agent in The Flood (season 6, ep 5) when the Upper East Side apartment falls through. The agent followed it with, "That's the only way to think about these things."
  17. Di really sounds like Rachel Menken. She also looks a bit like Midge.
  18. Don accidentally turns her into a prostitute: other than his whole HISTORY with that, this is an echo of when he gives Sylvia money in Collaborators (season 6, episode 3).
  19. Pete talks about how Clara is pregnant by another employee, who will probably just "look the other way." WELL YOU SHOULD KNOW PETE
  20. Pete mentions Fillmore Auto Parts, who were a part of The Beautiful Girls (season 4, episode 9) in two big ways: 1) their racist hiring policies stir up trouble between Peggy and Abe and in the SCDP office, and 2) Joan etc. have to smuggle out Miss Blankenship's body during Don's meeting with them.
    Pete says they're no longer clients. Wonder what happened there...
  21. Pete has to buy an apartment for tax purposes, like Peggy does in Season 6.
  22. Joan goes on her shopping spree at Bonwit Teller, where she briefly worked to support her terrible husband in Season 3 (specifically, episode 8, Souvenir).
  23. Joan is trying on a black dress with a deep v-neck and similar v-shape in the back. She has the saleswoman unzip the dress at the end of the scene. We've seen her wear a similarly-cut dress, with unzipping also being a key part of the scene: her liaison with Herb in The Other Woman (S5, E11).
  24. The shiva echoes the fur casting: the row of coats looks like the row of furs at the casting, but of course the mirrors are covered.
  25. Rachel has a son and a daughter, like Don did when they met.
  26. The women in Don's life die of cancer. Rachel died of leukemia. Anna died of an unspecified cancer. So did his stepmother (stomach cancer). Betty had a cancer scare in season 5 (episode 3, Tea Leaves).
  27. Peggy talking about quitting a job because she wanted to go to Paris is a reference to The Other Woman (season 5, episode 11), when Don throws the money in her face and she starts looking for other jobs.
  28. An impromptu trip to Paris--also not taken--is proposed by Don to Midge after his unexpected $2500 bonus from Cooper in The Hobo Code (season 1, episode 8).
  29. Stevie says Peggy is old-fashioned--callback to Christmas Comes But Once a Year (season 4, episode 2) when she's working with Freddy on Pond's Cold Cream and she gets called old-fashioned by Mark. She turns around and calls Freddy old-fashioned.
    And THAT episode calls back to The Jet Set (season 2, episode 11) when Kurt cuts her hair and says, "You are old style."
  30. Peggy's sarcastic "It's where margarine was invented" is a callback to Man With a Plan (season 6, episode 7), when she surprises and delights Ted by knowing that margarine was invented for Napoleon's army.
  31. Nixon's speech: Nixon tracks through the series in a big way--the first season has the agency working on his campaign, and Don watches his 1968 campaign ads at home in Season 6.
  32. In general: it's nearly precisely 10 years later (series started in May 1960, now it's April 1970). Don and Roger are up to their same tricks. The guys from McCann could be straight out of 1960.
    As Don says in Out of Town (season 3, episode 1): "I keep going to new places and ending up somewhere I've already been."