The Hebrew word "נֶחְתַּ֥ךְ" (neḥ·taḵ), translated as "decreed" in the NASB, means to be determined, be settled, be marked out. However, we look for a decree to indicate the start of the time period based on the following verse:
Know therefore and understand, that from the going forth of the
commandment to restore and to build Jerusalem unto the Messiah the
Prince shall be seven weeks, and threescore and two weeks: the street
shall be built again, and the wall, even in troublous times. (Daniel
9:25)
That commandment, recorded in Ezra 7, was most fully given by Artaxerxes in his seventh year (vs. 7). The command broadly applied to the rebuilding of Jerusalem, but especially was the temple's rebuilding a focus of this commandment.
Whatsoever is commanded by the God of heaven, let it be diligently
done for the house of the God of heaven: for why should there be wrath
against the realm of the king and his sons? (Ezra 7:23)
Because the reign of Artaxerxes is well established in history, the date of this proclamation is known to have been in 457 B.C. However, if we understand the prophecy, we can calculate back to that date ourselves from its endpoint.
Prophetic years in the Bible were represented by days. The book of Daniel was written in multiple languages, but this part is actually Hebrew, and in Hebrew the word for "day" ("יוֹם", yowm) could also mean "time" or "year." So it was natural for the Hebrew mindset to link the concepts of day and year. And God used this parallelism to apply His prophetic times, where He appoints a day to represent a year.
After the number of the days in which ye searched the land, even forty
days, each day for a year, shall ye bear your iniquities, even forty
years, and ye shall know my breach of promise. (Numbers 14:34)
For I have laid upon thee the years of their iniquity, according to
the number of the days, three hundred and ninety days: so shalt thou
bear the iniquity of the house of Israel. And when thou hast
accomplished them, lie again on thy right side, and thou shalt bear
the iniquity of the house of Judah forty days: I have appointed thee
each day for a year. (Ezekiel 4:5-6)
These verses demonstrate the Biblical day-for-year principle that is used in prophecy, and in Revelation parallel times are given showing the same principle. When we apply this to the prophecy given in Daniel 9, the math is simple.
70 weeks = 490 days --> 490 years
But the last week is "cut off" from this time period, or set apart.
Know therefore and understand, that from the going forth of the
commandment to restore and to build Jerusalem unto the Messiah the
Prince shall be seven weeks, and threescore and two weeks: the street
shall be built again, and the wall, even in troublous times.
And after threescore and two weeks shall Messiah be cut off, but not
for himself: and the people of the prince that shall come shall
destroy the city and the sanctuary; and the end thereof shall be with
a flood, and unto the end of the war desolations are determined. (vss.
25-26)
Seven weeks and threescore and two weeks = 69 weeks
(Remember that a score is 20.)
69 weeks = 483 days --> 483 years
From the time of the commandment to the coming of the Messiah was to be 483 years. In the midst of that last week (see vs. 27, below) that is set apart from the the others (70 - 1 = 69), the Messiah is to be "cut off." That's a Hebrew euphemism for being killed.
And he shall confirm the covenant with many for one week: and in
the midst of the week he shall cause the sacrifice and the oblation to
cease, and for the overspreading of abominations he shall make it
desolate, even until the consummation, and that determined shall be
poured upon the desolate. (Daniel 9:27)
When Jesus was crucified, that ended the law of sacrifices. The veil in the temple was supernaturally rent, top to bottom, to indicate this, as described in the gospels.
And, behold, the veil of the temple was rent in twain from the top to
the bottom; and the earth did quake, and the rocks rent; (Matthew
27:51)
And the veil of the temple was rent in twain from the top to the
bottom. (Mark 15:38)
And the sun was darkened, and the veil of the temple was rent in the
midst. (Luke 23:45)
This was prophesied to occur "in the midst of the week." Since one week represents seven years of time, the middle of the week would have three and a half years to either side. Jesus ministered for three and a half years. Following this, there were yet three and a half years' time granted to the Jews until A.D. 34 when the gospel went from them to the Gentiles, as marked by their rejection of it at the stoning of Stephen.
As I understand the prophecy, it was Christ's atonement, Christ's blood, which "anointed" the Most Holy Place.