Royal assent is not sufficient to give legal effect to an Act of Tynwald. By ancient custom, an act did not come into force until it had been promulgated at an open-air sitting of Tynwald, usually held on Tynwald Hill at St John's on St John's Day (24 June), but, since the adoption of the Gregorian calendar in 1753, on 5 July (or on the following Monday if 5 July is a Saturday or Sunday). Promulgation originally consisted of the reading of the Act in English and Manx; but, after 1865 the reading of the title of the act and a summary of each section were sufficient. This was reduced in 1895 to the titles and a memorandum of the object and purport of the act, and, since 1988, only the short title and a summary of the long title have been read.

Under ancient custom, an Act of Tynwald didn't go into effect until decreed at which location?