The celebration was supposed to be held Friday, but after a record-high 1,701 sea lions in October, the animals fled without warning the following month.
Jim Oswald, a spokesman for the Marine Mammal Center, said there were about 20 animals out on the dock Wednesday. The marine center has been monitoring the sea lions since they first appeared in 1990.
"It's going to be a scaled back event," Oswald said of Friday's anniversary celebration.
He said docents from the Marine Mammal Center would be available to educate guests about the animals, but Pier 39 will wait until some of the sea lions return before throwing them a party.
Oswald said it could take anywhere from weeks to months for the sea lions to return, depending upon food sources and other environmental factors.
The sea lions first came to Pier 39's "K" dock in large numbers in mid-January of 1990, when about 150 of them appeared without warning, according to the Marine Mammal Center.
About 150 to 300 of them have been hauling out every year since. This year they came out in record numbers, according to Oswald.
During the summer, most of the sea lions go to breeding grounds in the Channel Islands, off the coast of Santa Barbara.
Then in the winter, they gather at the dock to take advantage of the area's abundant winter herring supply, according to the Marine Mammal Center.
Although no one knows why the sea lions began coming to Pier 39, experts suspect it is partly because great white sharks and Orcas, which feed on the 6- to 7-foot sea lions, do not come into the San Francisco Bay.