# Then it was the day when

"At the time when" or "One day when." This is not a specific day but apparently the gathering happened often.

# sons of God

This refers to angels, heavenly beings.

# to present themselves before Yahweh

"to stand together before Yahweh as he commanded them to do."

# Yahweh

This is the name of God that he revealed to his people in the Old Testament. See the translationWord page about Yahweh concerning how to translate this.