That's a tough one. Most of the better restaurants that are open for lunch are open only Mon-Fri. And most of the rest are not open for lunch at all.
Some South End places do brunch on Sat as well as Sun. Union, Gaslight, Aquitaine, etc. That doesn't sound very festive, however.
The places that are reliably open are hotel restaurants. The ones that do it up right include:
Bristol Lounge at Four Seasons Hotel
Henrietta's Kitchen at Charles Hotel
Eastern Standard at Commonwealth Hotel
Cafe at the Taj Hotel
The grand dining room at the Langham (Julien) has closed. Only the Cafe Fleuri remains, and does not feel like a special place.
Now if you could wait until dinner....
We ended up walking around the Back Bay and made our way to the South End and had a nice late lunch at Petit Bistro Robert. The food, wine and service were all great, except for the incredibly loud couple seated next to us. They apparently did not get the memo about close seating and being courteous to your neighbors. I was with my best guy, and we found parking in both places. Added bonus.