On the federal holiday honoring the late renowned civil rights hero, President Joe Biden forgot the name of Dr. Martin Luther King Jr.'s daughter-in-law when singing "Happy Birthday" to her on Monday.

During his speech at the National Action Network's Martin Luther King Jr. Day breakfast, 49-year-old Arndrea Waters King was honored with the MLK Visionary Award.

"Martin the third, we celebrate a legacy of your beloved father and mother. They worked for the beloved community. But congratulations today to the honorees, including your wife, who I understand it's her birthday today?" Biden said.

Before singing the birthday song, Joe Biden explained that his wife has a rule in her family that if it is someone's birthday, you should sing "Happy Birthday."

"You ready?" He asked.

Martin Luther King III, age 65, and his wife led the crowd in singing "Happy Birthday" at the party hosted by Al Sharpton's National Action Network. Arndrea's name came out sounding like "Valz-dvit" after what seemed to be a moment of stuttering on Joe Biden's part, according to Yahoo.

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It is unclear if the president apologized to the woman, but those at the Mayflower Hotel in Washington, D.C. appeared to laugh and shake their heads in surprise.

Martin Luther King III and Arndrea have been married since 2006, and they have one daughter, Yolanda Renee King.

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New Joe Biden Video Is Just One of His Many Gaffes

To save face, Biden tried to console his middle-aged honoree by saying, "Well, it's hell turning 30. But you have to put up with it."

As the oldest president in history, Biden's mental capacity is often questioned; nevertheless, his supporters insist that the president is simply prone to gaffes and stutter, New York Post noted.

For instance, despite officially mourning Rep. Jackie Walorski's (R-Ind.) death and calling her family to offer his condolences in August, the president surprised viewers the following month by asking, "Where is Jackie?" as he looked for the deceased lawmaker.

On at least six occasions, most recently on January 5, Joe Biden called his vice president, Kamala Harris, "President Harris."

Biden has already announced his candidacy for reelection in 2024. If he finishes his second term in January 2029, he will be 86.

Joe Biden Slams Republicans in His Speech

During his speech on Monday, the president bragged about cutting the federal debt by $1 trillion while attacking Republicans, The Daily Mail reported.

"Hear me, that's a fact," Biden said. "And there's going to be hundreds of billions reduced over the next decade."

After that, he mumbled something about the GOP and started to slur his words: "But so what, these guys - well, they're fiscally demented, I think."

On Sunday, he made history by becoming the first sitting president to preach at the same church where Dr. Martin Luther King Jr. served as pastor from 1960 until he died in 1968: Ebenezer Baptist Church in Atlanta, Georgia.

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Written by: Bert Hoover

WATCH: President Biden delivers remarks on MLK Day - From ABC News