"Lonely Days" is a ballad written and performed by the Bee Gees. It appeared on their album 2 Years On, and was released as a single, becoming their first Top Five hit in the US, peaking at number three in the Billboard Hot 100 and reaching number one in the Cashbox and Record World charts.

On Friday, 21 August 1970, the three Gibb brothers announced they would reunite and start recording together, nearly 16 months after Robin quit the group. They said later that they wrote "Lonely Days" and "How Can You Mend a Broken Heart" at their first reunion session, but the exact day when they recorded the song is unknown. However, a tape of stereo mixes received at Atlantic in October bears the tantalizing notation "August 20, 1970" which, if true, means the brothers announced the reunion the day after it happened. According to Robin Gibb in a 2001 Billboard interview with the Bee Gees, "That was written on Addison Road in Holland Park in London, in the basement of Barry's place".

This song was sung by all three together to Maurice's piano and bass and Bill Shepherd's string and horn arrangement, the slow verses contrasting with the pounding chorus. "'Lonely Days' was written in ten minutes. It was that quick. I was at the piano ten minutes". Barry revealed later in 1998, "A manager we had about five years back heard 'Lonely Days' in a restaurant and he said to a friend, 'That's one of my favorite Beatles songs' And he was managing us!"

Atlantic Records president Jerry L. Greenberg recalled:

"I heard that song ['Lonely Days'] and I went crazy. I thought it was going to be an amazing number one record. In those days we had our own recording studio, right down the hall from the main offices. Now the record was pressed and ready to go, but what I did was, I made up about 30 tape copies, just put the song on a tape on a regular plastic reel, and I called up a bunch of my promotions department friends."

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