Darillium

Darillium was a planet known for its Singing Towers. It had a long orbit; in human term, a night lasted 24 years. The Twelfth Doctor took River Song to see these towers the last time she saw him before she went to the Library and met the Tenth Doctor. (TV: The Husbands of River Song, Forest of the Dead)

In the TARDIS, an older Eleventh Doctor and River Song stumbled upon his younger eleventh self before visiting Darillium, and the two Doctors briefly discussed that it was her last meeting with them. (HOMEVID: Last Night) Ultimately unwilling to go to Darillium yet, the Doctor cancelled his plans at the last minute, and backed out of this date several times. (TV: The Husbands of River Song)

River told the Doctor in the Library that "the last time I saw you - the real you, the future you, I mean - you turned up on my doorstep with a new haircut and a suit. You took me to Darillium to see the Singing Towers. What a night that was. The towers sang and you cried. You wouldn't tell me why, but I suppose you knew it was time. My time. Time to come to the Library. You even gave me your screwdriver. That should have been a clue." (TV: Forest of the Dead)

Darillium Darillium