Complex sentence: The World Youth Day began with a pre-program in numerous Canadian dioceses from 18 to 21 July, to which the international guests were also invited to stay in 35 dioceses throughout Canada (mostly in Montreal and Quebec).
Split into simpler sentences: The World Youth Day began with a pre-program in numerous Canadian dioceses. From 18 to 21 July, the international guests were also invited to stay in 35 dioceses throughout Canada (mostly in Montreal and Quebec).