Daniel 9:2
In the first year of his reign I Daniel understood by books the number of the years, whereof the word of the Lord came to Jeremiah the prophet, that he would accomplish seventy years in the desolations of Jerusalem.

Daniel prayed and mourned for the destruction of the Temple of Solomon: "cause thy face to shine upon thy sanctuary that is desolate".

Below is the prophecy of Jeremiah:

Jeremiah 29:
4 Thus saith the Lord of hosts, the God of Israel, unto all that are carried away captives, whom I have caused to be carried away from Jerusalem unto Babylon;
5 Build ye houses, and dwell in them; and plant gardens, and eat the fruit of them;
6 Take ye wives, and beget sons and daughters; and take wives for your sons, and give your daughters to husbands, that they may bear sons and daughters; that ye may be increased there, and not diminished.
7 And seek the peace of the city whither I have caused you to be carried away captives, and pray unto the Lord for it: for in the peace thereof shall ye have peace.

Jeremiah 29:10
For thus saith the Lord, That after seventy years be accomplished at Babylon I will visit you, and perform my good word toward you, in causing you to return to this place.

While Daniel was praying, the Angel Gabriel came and spoke the following words to the Prophet:

Daniel 9:24
Seventy weeks are determined upon thy people and upon thy holy city, to finish the transgression, and to make an end of sins, and TO MAKE RECONCILIATION for iniquity, and TO BRING EVERLASTING RIGHTEOUSNESS, and to seal up the vision and prophecy, and TO ANOINT THE MOST HOLY.

The Prophecy of Jeremiah is very clear about the length of the Captivity in Babylon, Seventy Years. To give further insights to Daniel about what will happen farther in time, the Angel spoke about 70 weeks of years (70 x 7 = 490 years).

It is important to understand what the Angel means when he referred to "Reconciliation for Iniquity":

Leviticus 8:15
And he slew it; and Moses took the blood, and put it upon the horns of the altar round about with his finger, and purified the altar, and poured the blood at the bottom of the altar, and sanctified it, TO MAKE RECONCILIATION upon it.

2 Chronicles 29:
23 And they brought forth the he goats for the sin offering before the king and the congregation; and they laid their hands upon them:
24 And the priests killed them, and THEY MADE RECONCILIATION with their blood upon the altar, to make an atonement for all Israel...

The Angel Gabriel also spoke about 70 Weeks to bring everlasting righteousness. Before the Temple of Solomon was destroyed, when it was consecrated, the Children of Israel sang: for he is good; for his mercy endured for ever.

Psalm 103:
17 But THE MERCY OF THE LORD IS FROM EVERLASTING TO EVERLASTING upon them that fear him, and HIS RIGHTEOUSNESS unto children's children;
18 To such as keep his covenant, and to those that remember his commandments to do them.

2 Chronicles 5:13
It came even to pass, as the trumpeters and singers were as one, to make one sound to be heard in praising and thanking the Lord; and when they lifted up their voice with the trumpets and cymbals and instruments of musick, and praised the Lord, saying, FOR HE IS GOOD; FOR HIS MERCY ENDURETH FOR EVER: that then the house was filled with a cloud, even the house of the Lord;

2 Chronicles 7:3
And when all the children of Israel saw how the fire came down, and the glory of the Lord upon the house, they bowed themselves with their faces to the ground upon the pavement, and worshipped, and praised the Lord, saying, FOR HE IS GOOD; FOR HIS MERCY ENDURETH FOR EVER.

Again in the book of Ezra it is reported: when the Temple of Jerusalem was being rebuilt, the Children of Israel sang together: because he is good, for his mercy endureth for ever.

Ezra 3:11
And they sang together by course in praising and giving thanks unto the Lord; BECAUSE HE IS GOOD, FOR HIS MERCY ENDURETH FOR EVER. And all the people shouted with a great shout, when they praised the Lord, because the foundation of the house of the Lord was laid.

The most important point to understand, is that 70 Weeks of years after the consecration of the Temple of Solomon (490 years later), the second Temple was completed. You do remember that the Angel Gabriel also told Daniel that 70 weeks are determined "to anoint the most holy".

The message of the Angel is: after 490 years, the Temple (the most Holy) with the Holy Altar and the Holy of Holies would be rebuilt and anointed during its consecration:

Exodus 30:
26 And thou shalt anoint the tabernacle of the congregation therewith, and the ark of the testimony,
27 And the table and all his vessels, and the candlestick and his vessels, and the altar of incense,
28 And the altar of burnt offering with all his vessels, and the laver and his foot.
29 And thou shalt sanctify them, that they may be MOST HOLY...

Numbers 7:88 ... This was the dedication of the altar, after that it was anointed.