Seven, as in Revelation 15:1, signifies everything in an universal sense. The number 'seven' was considered holy, as is well known, because of the six days of creation, and the seventh, which is the celestial self, where peace, rest, and the Sabbath is. The number seven occurs so frequently in the rites of the Jewish church and is held holy everywhere.

So times were divided into seven, longer and shorter intervals, and were called weeks, like the great intervals of times till the coming of the Messiah, in Daniel 9:24-25. The time of seven years is called 'a week' by Laban and Jacob, as in Genesis 29:27-28. So wherever the number seven occurs, it is considered holy and sacred, as in Psalm 119:164, and in Isaiah 30:26.

As the periods of a person's regeneration are distinguished into six, prior to the seventh, or the celestial self, so the times of vastation are also distinguished, until nothing celestial is left. This was represented by the many captivities of the Jews, and by the last Babylonian captivity, which lasted seven decades, or seventy years. This was also represented by Nebuchadnezzar, in Daniel 4:16, 22, 29. It also refers to the vastation of the end times, in Revelation 15:1, 7-8. They should 'tread the holy city under foot, forty and two months, or six times seven,' as in Revelation 11:2 and Revelation 5:1. So the severity and increments of punishment were expressed by the number seven, as in Leviticus 26:18, 21, 24, 28 and Psalm 79:12.

(References: Apocalypse Explained 5, 7-8, 15; Arcana Coelestia 395; Daniel 9, 9:24, 9:25; Psalms 119)

15. And keep the things which are written therein, signifies from the love of truth. This is evident from the signification of "observing" and "keeping," as being to perceive, to will, and to do according thereto, here according to the doctrine of heaven; and from the signification of "the things which are written therein," as being, from the love of truth, or from the delight of that love; for what is done from love is done from delight; delight is from no other source. This is signified by "the things which are written therein," because the things contained in the doctrine of heaven are written on the heart, and thus on the life, of those here described; and things written on the heart and life are written on the love; for "heart" in the Word signifies love (Arcana Coelestia 7542, 9050, 10336). The love of truth is meant, because these things are said of the doctrine of heaven, and the doctrine of heaven is from truths. In the Word frequent mention is made of observing and keeping the precepts, the commandments, the words, and the law: and by observing and keeping is there signified to understand, to will, and to do, as in Matthew:

Teaching them to observe all things whatsoever I commanded you (Matthew 28:20);

in Luke:

Blessed are they that hear the Word of God and keep it (Luke 11:28);

in John:

Verily, verily, I say unto you, if anyone keep My word he shall never see death (John 8:51);

in the same:

If any one love Me he will keep My word: He that loveth Me not, keepeth not My words (John 14:15, 23-24);

and in the same:

If ye keep My commandments ye shall abide in My love. Ye are My friends if ye do the things which I command you (John 15:10, 14).

To do is to will, and to will is to do; because in deeds the will is everything.