the Church does not have any position on how or when Catholics decorate their home. Catholics who observe the liturgical seasons celebrate Advent in for the 4 weeks preceeding Christmas, with Advent wreath and various prayers and practices intended to foster penance and hopeful anticipation. The put up decorations, bake, shop and other activities in preparation for the Holy Day in that same spirit, and according to the needs of their families.
If you are asking about decorating the Church, the colors proper to the liturgical seasons must be observed, Advent and Lent as penitential seasons means decorations and flowers are absent or limited. For practical reasons it may be necessary to put up trees (if used) before the 4rd Sunday of Advent so workers do not disrupt Sunday Masses, the the actual lights would not be turned on until that evening or later, and poinsettias and other flowers not brought in until then. The empty stable would be appropriate for the 4th Sunday of Advent (again, if necessary to erect it early because of the schedule of the workers). But Mary and Joseph would not appear until Christmas eve and of course the Babe for the anticipated Masses that day.