Washington National Cathedral plans to announce Thursday that it has received a $5 million donation for earthquake repairs from the Lilly Endowment, the Indianapolis-based foundation created by the family that built the Eli Lilly pharmaceutical company.

The announcement is scheduled for the first anniversary of the 5.8-magnitude quake that struck the Washington region last Aug. 23, and inflicted what is now estimated to be $20 million in damage on the cathedral.

Cathedral officials said they will mark the anniversary Thursday with the first major repair on the mammoth limestone structure in Northwest Washington — the replacement of a marred stone ornament called a crocket.

They also plan prayers, blessings, and a ceremonial pealing of the cathedral bells.

Millions of dollars have been spent on the cathedral in the months since the earthquake, but that money went for stabilization of the structure and preparation for upcoming work, officials said.

The cathedral expressed its deep gratitude to the Lilly Endowment, noting that $2.4 million of the previously raised $2.8 million had been spent.

“The gift . . . makes an enormous amount of difference in our ability to restore this property,” the Rev. Francis H. Wade, interim dean of the cathedral, said Wednesday in an interview.

“The Lilly family has been an important part of this cathedral for a long time,” he said, noting that the family helped fund construction of the cathedral’s northwest tower in decades past. “They’ve been enormously helpful to us.”

Wade said that progress in the past year has been extensive but that plenty still needs to be done.

“The road is long,” he said. “I think when it’s finished there will be rejoicing in heaven, and I also think that’s where I’ll be doing my rejoicing.”

Wade, 71, said that if all the money for repairs were in hand, the work would take five years. But that is not the case, for now, and the cathedral’s chief stonemason has estimated that the work could take a decade.

The 301-foot-tall Episcopal cathedral, which took 83 years to build, was rattled by the earthquake. Huge stone finials, gargoyles and crockets shook loose and came crashing down.

The cathedral, which was completed in 1990, was closed for almost three months while scaffolding and safety netting were put up, unstable parts of the structure were secured, and inspectors tallied the damage.

Buildings across the area were shaken by the quake, which was centered in Louisa County, Va., and which sent thousands of bewildered residents scurrying.

The Washington Monument was extensively damaged, and remains closed for repairs that could take until 2014 to complete.

District, state and federal officials are hosting a one-year review of the event Thursday morning near the monument.

At the cathedral Thursday, stone carvers will install the first of the so-called “dutchmen,” newly made replacements that will be glued and pinned in spots where damaged elements have been lost.

On Wednesday morning, the chief stonemason, Joseph Alonso, and stone carver Sean Callahan, showed the first big dutchman — carved by Callahan out of a hunk of Indiana limestone salvaged from the building. It is a 60-pound crocket, one of the ornate stone representations of curled leaves that decorate much of the cathedral.

The new crocket will be fastened with heavy-duty glue and stainless-steel dowels and staples into an alcove carved into a damaged pinnacle on the soaring central tower.

Callahan said the epoxy glue is stronger than the internal structure of the stone, which would break before the glue gave way.

The crocket was about 95 percent finished, he said, and the final touches will be done once it is in place.

He hoisted the pure white stone into its alcove to show how it fit. It fit perfectly.

“I’m happy with it,” Callahan said. “You’re always nervous you’re going to chip something before you get it finally set. That’s what I don’t want to have happen.”

Asked how many more such repairs will be needed, Alonso gestured across the forest of stone pinnacles and finials.

“Hundreds,” he said.