Seven is the number of completion. The creation was completed in seven days (including the day of rest).
Revelation 4:5 speaks of the "seven Spirits of God", and "seven churches". These are references to the completeness and holiness of God and the whole church being present before Him.
The final judgments on the earth are a set of three sevens. Complete and ultimate judgment. These take place during seven "days" (probably prophetic days, that is, years), the complete time allocated for this judgment. This is the completion of the shadow seen before the Flood (the first judgment) - Genesis 7:4 "For yet seven days, and I will cause it to rain upon the earth forty days and forty nights; and every living substance that I have made will I destroy from off the face of the earth."