In its present form, the custom of displaying figures depicting the birth of Jesus Christ owes its origin to St. Francis of Assisi, who made the Christmas crèche or manger for Christmas Eve of 1223.
The blessing of the Christmas manger or nativity scene may take place on the Vigil of Christmas or at another suitable time.When the manger is set up in the home, it is appropriate that it be blessed by a parent or another family member. (Source: USCCB)
All make the sign of the cross as the leader says:
Our help is in the name of the Lord.
R/. Who made heaven and earth.
One of those present or the leader reads a text of sacred Scripture, for example, Luke 2:1 (lines 1-8) or Isaiah 7:10 (lines 10-15, the birth of Emmanuel).
Reader: The Gospel of the Lord.
R/. Praise to you, Lord Jesus Christ.
The leader prays with hands joined:
God of every nation and people,
from the very beginning of creation
you have made manifest your love:
when our need for a Savior was great
you sent your Son to be born of the Virgin Mary.
To our lives he brings joy and peace,
justice, mercy, and love.
bless all who look upon this manger;
may it remind us of the humble birth of Jesus,
and raise our thoughts to him,
who is God-with-us and Savior of all,
and who lives and reigns forever and ever.
—From "Catholic Household Blessings & Prayers"