Youkali was the home world of the Youkalians. It was later an Earth colony.
Circa 5000 BC, the Osirans and Sutekh battled on Youkali. (PROSE: Return of the Living Dad)
In 2257, Youkali sent a medical shuttle to Yemaya 4. (PROSE: Sleepy)
In 2587, Bernice Summerfield and Jason Kane visited Youkali on an archaeological expedition. (PROSE: Return of the Living Dad)
In 2982, there was a hostage situation on Youkali. (PROSE: So Vile a Sin)
At some point, the Doctor spent a week cleaning up the after-effects of Daniel Joyce's visit to Youkali. (PROSE: Unnatural History)