Jennifer Aniston and John Mayer
Lester Cohen/WireImage; National Photo Group
John Mayer has prepared his own serenade to help his girlfriend celebrate the big 4-0.
"He did write a special song just for her for her birthday," a source tells PEOPLE of the Grammy winner. No word yet on his performance!
Think you know Jen well enough to write your own birthday tune? Take PEOPLE.com's Jennifer Aniston birthday quiz!
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