KWAJALEIN, Marshall Islands, Sunday, Aug. 22— Talk about a lost weekend!

The nearly 3,000 Americans living on this remote Pacific atoll have a good excuse for not remembering Saturday night: there wasn't one.

Residents were going to bed Friday night and waking up Sunday morning because at midnight -- 8 A.M. Eastern daylight time on Saturday -- Kwajalein was jumping from one side of the international date line to the other.

The Marshall Islands, a group of about 100 islets of which Kwajalein is the largest, sit west of the international date line. But Kwajalein, which is about 300 miles west of the line, had synchronized its day of the week with the United States mainland, to the east, about 40 years ago when the United States Army established a missile test range here.

The Republic of the Marshall Islands requested the latest change so all its islets would be on the same side of the date line.

Kwajalein's workweek will shift to Tuesday through Saturday, the mainland's Monday through Friday. Church services will still be held on Sunday, which will seem like Saturday as it is the first weekend day off. Many people plan to use their Mondays to run errands like most mainlanders do on Saturdays.

"It really kind of makes you daffy," a nurse, Celeste Kim, said Friday. "You're not sure if you're coming or going."

Ms. Kim added that national holidays, like the coming Labor Day, will be on Saturdays, which are Fridays in the United States, instead of Tuesdays, which would be Mondays.

Kwajalein has about 3,000 residents -- American military and civilian workers; there are virtually no native islanders. It has been a missile testing site for the United States Army. The 4,200-mile distance from Vandenberg Air Base in California was important during the cold war -- it was roughly as far away as Russia is from the United States.

Residents marked the occasion with a two-mile, three-day run, "Run Around the Clock," starting just before midnight Friday, and ending a short time later, on Sunday morning.

Others who are not as active may wonder what happened to their weekend, said Christopher Powell, a senior police patrol officer.

"I'm sure there will be some wild parties tonight and people will have been drinking all night," he said Friday. "And then they will wake up and won't even realize they have missed Saturday."