The 9th U.S. Circuit Court of Appeals said a three-judge panel will publish its opinion on Tuesday on whether the ban violates the constitutional rights of gays and lesbians.
The panel heard arguments more than a year ago but delayed its decision to seek guidance from the California Supreme Court on whether Proposition 8 supporters had legal authority to challenge a trial judge's ruling that the voter-approved measure was unconstitutional.
Even if the appeals court upholds the lower court on Tuesday, same-sex marriages are not likely to resume in California in the near future.
Proposition 8 supporters and opponents have said they would appeal to the U.S. Supreme Court if they lose in the immediate court.
Supporters planned to gather at West Hollywood and Los Angeles city hall hoping one of the most liberal federal courts in the country will give them reason to celebrate.
Some gay rights advocates worry that a loss could set the movement back decades.
"It's clear that there are four people who would probably vote with us and four people who would probably vote against us, and that means it's all going to come down to how Justice (Anthony) Kennedy will vote," said Lorri L. Jean, CEO of the L.A. Gay & Lesbian Center.
Andy Pugno, author of Proposition 8, is hoping the Supreme Court will allow Californians to make the decision.
"A win for us would be a Supreme Court decision saying this is not something that judges should decide, this is something the people and their elected representatives should decide on a state-by-state basis," Pugno said.
"The court that has heard this case is more progressive and so I'm hoping for the best possible result," said attorney Gloria Allred, who is advocating for gay marriage. "As far as I'm concerned, the best possible result is to uphold the right to marry."
The decision is expected to come down about 10 a.m. Tuesday.
The Associated Press contributed to this story.