But the imminent arrival of a barbaric tyrant at Downton is casting a pall over the couple's big day.
Oh, wait, that's just Lady Mary's other imposing grandmama, the one from America, Martha Levinson (Shirley MacLaine). Batten down the china! Get some smelling salts for the Dowager Countess (Maggie Smith)!
Aside from the titanic meeting of those ever-so-Grandes Dames, what else is happening as Season 3 gets under way?
Poor Bates (Brendan Coyle) is languishing in gaol, leaving devoted Anna (Joanne Froggatt) the only person still trying to prove his innocence.
The lovely Lady Sybil (Jessica Brown Findlay) and Tom Branson make their first visit to Downton as a married couple. The former chauffeur and ardent Irish Republican quickly discovers why politics is not a suitable topic for discussion at the dinner table.
Lady Edith (Laura Carmichael) is more determined than ever not to be the family spinster. Those downstairs serpents, Thomas (Rob James-Collier) and Mrs. O'Brien (Siobhan Finneran), have turned on each other.
And Lord Grantham (Hugh Bonneville) has discovered, to his vast chagrin, that the old Victorian cliche "safe as houses" doesn't always hold true.
Those tidbits are just hors d'oeuvres, cucumber sandwiches before the main course. For better or worse, you can find all the plot details you want online. The season we are about to enjoy has just finished airing in the United Kingdom, and websites on both sides of the pond have been buzzing this week about the cast defection that leads to the shattering Christmas-at-Downton finale.
All this feverish anticipation for a Masterpiece Classic production? Very un-PBS-like.
But Downton Abbey has come to fascinate American viewers to a degree almost unheard of for an imported period drama since Upstairs Downstairs in the 1970s and Brideshead Revisited in the 1980s. Its massive popularity has taken almost everyone by surprise.
"Candidly, we didn't expect it to be such a breakout hit," says Rebecca Eaton, executive producer of Masterpiece. "The ratings for the first episode [in 2011] were high and they stayed high over the next several weeks. That was interesting. It's a serial and people were coming back."
It quickly became apparent that Downton was getting attention well beyond the usual Charles Dickens and Jane Austen crowd.
"There started to be buzz in social media," Eaton says. "Over and over I was reading, 'I'm just addicted to Downton Abbey.' 'I'm obsessed with Downton Abbey.' Those are young people's phrases."
Fanned by enthusiastic word of mouth, the audience began to swell. Ratings for the second season rose by 25 percent, with 5.4 million tuning in for the finale.
Masterpiece viewership among 18-to-34-year-old male viewers rose by 111 percent; 251 percent for women in the same age bracket. Online traffic at PBS's digital download site was up by 400 percent.
It was widely reported in November that first lady Michelle Obama had, upon request, been shipped advance copies of Downton's Season 3.
But Masterpiece's Eaton acknowledges she has been besieged by celebrities of all stripes. "Major Hollywood figures, very, very wealthy people, literati - as well as endless relatives," she says and laughs.
Many PBS stations are offering catered screening parties on Sunday, inviting ticketholders to attend in period dress. The soiree at WHYY's studios in Philadelphia sold out hours after it was announced.
Lesley Nicol, who plays Downton's delightfully dowdy cook, Mrs. Patmore, is virtually unrecognizable when she's out of character. "It gets quite hot in the kitchen," she says. "I don't get any makeup at all and the hotter and sweatier I get, the more they like it."
But viewers have become so spellbound by the show, she reports, that during a visit to the United States, she was recognized again and again. Purely by her voice.
"I was just in Los Angeles in a Starbucks and the young boy who served me coffee said, 'Are you in Downton Abbey?' and I said, 'Yes. I am.'
"I was in Petco buying stuff for my animals and the guy ringing me up said, 'Is that you, Mrs. Patmore?' "
Why is this archaic soap opera resonating so deeply with contemporary Americans? Seth Koven, a history professor at Rutgers University, maintains that we have been conditioned to embrace it.
"There's a long and well-established audience for British country-house dramas," he says. "The first on TV was The Forsyte Saga and then Brideshead Revisited. In Hollywood, it goes back to huge box office hits like Mrs. Miniver, a film which is used by Downton Abbey crudely and explicitly. There's a good 60 years of preparing an American audience for Downton Abbey.
"The whole idea of country houses is not an accident," Koven continues. "It was the product of a deliberate policy choice by people in Britain. They recognized it as a valuable social and economic tool. They use it to attract American tourists. We have been trained to love British country houses. Americans have long had a sense of misplaced nostalgia for a past that was never ours."
As Noel Coward wrote in 1938: "The stately homes of England/We proudly represent/We only keep them up for/ Americans to rent."
Perhaps Downton also succeeds because it pours its old, full-bodied port from a bottle far newer than it appears.
"It's a classic period-costume drama, but it's not an adaptation," says Eaton. "It's written for TV, which means [ Downton creator] Julian Fellowes can pace the dialogue and structure the scenes to match modern sensibilities. The way he creates tension between the characters keeps things unresolved and places cliffhangers at the end of episodes - it's very artful."
Were we in fact set up to fall in love with Downton Abbey? It hardly matters, because at this point, we'd follow the Crawleys over a cliff.
Please have Carson pack us a lunch.
Contact David Hiltbrand at 215-854-4552, firstname.lastname@example.org, or follow on Twitter @daveondemand_tv.