The Holy Land was where Vicki Pallister guessed, and the First Doctor tacitly allowed, that they had landed in Palestine in 1190 and discovered Richard the Lionheart. It was a generalised regional name for the setting of the Crusades on Earth. Some of the cities in the Holy Land were Jaffa, Jerusalem and Basrah. (TV: The Crusade [+]David Whitaker, Doctor Who season 2 (BBC1, 1965).)
A peasant from John's England, who had been sent to London in the 1970s by a time eddy, told the Third Doctor that Richard was in the Holy Land. (TV: Invasion of the Dinosaurs [+]Malcolm Hulke, Doctor Who season 11 (BBC1, 1974).)
Behind the scenes[]
The Crusade doesn't define "the Holy Land" in any meaningful way, and the novelisation completely avoids the term.