The Literal Commandment
"Speak to Aaron and to his sons, saying, 'Thus you shall bless the sons of Israel. You shall say to them:
The LORD bless you, and keep you;
The LORD make His face shine on you, And be gracious to you;
The LORD lift up His countenance on you, And give you peace.'
So they shall invoke My name on the sons of Israel, and I then will bless them." (Numbers 6:23-27)
The literal commandment is that the Aaronic priests should bless the sons of Israel using what is traditionally called "The Aaronic Blessing". How frequently this should be done is not specified in this passage.