Jennifer Garner is striking up the band for Reese Witherspoon's birthday.

To celebrate the "Legally Blonde" star's 42nd birthday, Jen felt full band geek, donning a marching band uniform and playing "Happy Birthday" on the saxophone.

"Hey, @reesewitherspoon! I don't know why you're on my mind today…..," she wrote along with a slew of emojis. She added the hashtag "Happy Birthday Friend."

In the hilarious video, Jen stays in full character, marching to the center of the video and pivoting. After finishing, she smiles pivots and marches away.

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Jen and Reese have been friends for years.

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Jen has previously said she's proud to be a "band geek." As a teenager, she played saxophone for the John Adams Middle School marching band.

In 2016, she told Seth Meyers that she named her alto saxophone "Sally the Sexy Saxophone."

She showed off her skills again last that year, playing "Mary Had a Little Lamb" for Vanity Fair.

Just last year, she posted a video of herself watching the Texas A&M marching band at a football game.

"Once a band geek, always a band geek," she captioned the video.

Jen's birthday, by the way, is next month. Reese, start preparing your response.