89 Therefore, when the office is revised, these norms are to be observed: a) By the venerable tradition of the universal Church, Lauds as morning prayer and Vespers as evening prayer are the two hinges on which the daily office turns; hence they are to be considered as the chief hours and are to be celebrated as such. b) Compline is to be drawn up so that it will be a suitable prayer for the end of the day. c) The hour known as Matins, although it should retain the character of nocturnal praise when celebrated in choir, shall be adapted so that it may be recited at any hour of the day; it shall be made up of fewer psalms and longer readings. d) The hour of Prime is to be suppressed. e) In choir the hours of Terce, Sext, and None are to be observed. But outside choir it will be lawful to select any one of these three, according to the respective time of the day.
Source: Sacrosanctum Concilium (Vatican.va)