When were triples called monads for the first time?

This is covered in this English.SE question. In short, people were not all very happy about the term "triple", and tried to come up with something better. Jean Bénabou suggested "monad" during lunch at a meeting in 1966, and it was quickly adopted; for example, it appears in the titles of Anders Kock's and Eduardo Dubuc's theses from 1967 and 1969 respectively, and the use of the term is alluded to in the Introduction of the "Seminar on triples and categorical homology theory" from 1969 and in Lawvere's "Ordinal sums and equational doctrines" from that volume. Peter May also convinced Mac Lane to use "monad" in his book, which helped to popularize the term.

P. T. Johnstone (who wrote several books on Topos Theory) gave a Category Theory lecture in which he said this was originally called 'the standard construction', then 'triples', and finally 'monads' -- but he provided the caveat that mathematicians in Montreal still adhere to the term 'triples', such that if you're in Montreal and want to discuss monads, you should probably bear this in mind.