A Tisket – A Tasket
Upon entering their rooms, houseguests find a beach bag filled with water, towels, directions to the beach and magazines. It gives them license to venture out on their own without consulting the hostess.

Spell It Out
By the pool, guests can help themselves to plentiful towels and crossword puzzles. "When people put away their Blackberries and do the crosswords, the weekend has been a success," says Langdon.

A Feast
for the Eyes
A casual Friday-evening dinner kicks off the weekend. An elegant cream-and-white palette unifies the coral, shells and candles. Friday's fête feels beachy, while Saturday's décor is pulled from the garden.

Feeling
Fresh
"Mint grows in our garden like weeds," says Landgon. "I put a fresh mint bouquet in every guest bathroom to make the room smell fresh—like a spa." Plus, it's more unexpected than flowers.

All Hands on Deck
"Guests always want to help," says Langdon, who notes special tasks that are easy to assign. O'Hea's daughter Meghan (center, below) and Langdon's nieces Jordan and Alex shuck corn poolside while others roll strands of seagrass around cloth napkins so that setting the table is a snap.

Room For 60
Two tables for 30 stretch out along Langdon's front porch. The setup recalls "dinner parties in some great foreign film where everyone sits at long tables, talks, drinks lots of wine and has a blast." Langdon and O'Hea stand near the buffet to greet guests, many of whom hail from North Carolina, her home state, and Chicago, his hometown.

Sing The Blues
Blue hydrangeas are Langdon's passion. She arranges them in an assortment of vessels, such as silver creamers and pottery teapots. "Use what you find in the garden or beach as decoration," she advises.

Tiki Time
On her birthday night, Langdon surrounds her Sag Harbor home with tiki torches, which makes for a dramatic entrance. She has accumulated scores of hurricane lamps and votive candles and uses only candlelight when entertaining to set a romantic, exciting mood.

Warming Up
With prep work done, Saturday evening begins with poolside cocktails featuring "Keith's Killer Concoctions"—Grey Goose vodka, pink lemonade and fresh key lime juice—which can be premixed. It's one of many elements the couple chooses to prepare in advance.

Rise And Shine
A breakfast buffet awaits houseguests in the morning. Langdon lays it out the night before, which allows the hosts to sleep in. (Though they don't!) The candlelit birthday went off without a hitch. "It was the most seamless, magical evening. Probably the most successful party I've ever had the pleasure to host."