JANIS - I would like to have the till "the sons of Levi do offer again an offering unto the Lord in righeousness" better explained. Has this already happened or is this something yet to come at the time of the 2nd coming of Christ.

JOEL - The quick answer to your question is, we don't know. Now here's the long answer.
Here is the scripture from which you quote:

"Upon you my fellow servants, in the name of Messiah I confer the Priesthood of Aaron, which holds the keys of the ministering of angels, and of the gospel of repentance, and of baptism by immersion for the remission of sins; and this shall never be taken again from the earth, until the sons of Levi do offer again an offering unto the Lord in righteousness." (D&C 13:1)

The sons of Levi are the male members of the tribe of Levi, descendants of Levi, the third son of the patriarch, Jacob. The nature of this offering that will be made by the sons of Levi is not specifically known. The Prophet Joseph Smith suggested that it might be the same burnt offering as of old:

"It is generally supposed that sacrifice was entirely done away when the Great Sacrifice was offered up, and that there will be no necessity for the ordinance of sacrifice in the future; but those who assert this are certainly not acquainted with the duties, privileges, and authority of the Priesthood, or with the prophets. These sacrifices as well as every ordinance, belonging to the priesthood, will when the Temple of the Lord is built, and the sons of Levi be purified, be fully restored and attended to in all their powers, ramifications, and blessings." (D. H. C. "Teachings of the Prophet," p. 172.)

"All the ordinances and duties that ever have been required by the Priesthood, under the directions and commandments of the Almighty in any of the dispensations, shall be had in the last dispensation." (History of the Church 4:210-11.)

If this is true then the offering will probably not be made until Christ appears in His temple at His second coming.

Many are of the opinion, however, that it will not be a burnt offering. They reason that the earlier sacrifices were in similitude of the coming sacrifice of Jesus Christ and that the coming of Christ ended the Mosaic law and the reason for performing the sacrifices.
So they propose another type of sacrifice which is suggested in the Doctrine and Covenants:

"For he is like a refiner's fire, and like fuller's soap; and he shall sit as a refiner and purifier of silver, and he shall purify the sons of Levi, and purge them as old and silver, that they may offer unto the Lord an offering in righteousness. Let us, therefore, as a church and a people, and as Latter-day Saints, offer unto the Lord an offering in righteousness; and let us present in his holy temple, when it is finished, a book containing the records of our dead, which shall be worthy of all acceptation. (D. & C. 128:24)

The "offering in righteousness" is here identified with temple work for the salvation of the dead, which encompasses all the principles of the plan of salvation. If understood in this sense, the offering is already being made in temples across the world by members of the Church.

On a separate issue, the way the D&C scripture is written, saying that the priesthood would remain "until" the sons of Levi make their offering; would suggest that the Aaronic priesthood is going to eventually be taken from the earth. But we need to consider all scriptures and other statements relating to this subject to understand what is meant.

In Mal. 3: 3 we read:
"And he shall sit as a refiner and purifier of silver: and he shall purify the sons of Levi, and purge them as gold and silver, that they may offer unto the Lord an offering in righteousness.' (See also the D&C 128:24 scripture above)

Oliver Cowdery, writing in the early days of the Church of the vision of John the Baptist, did not use the word "until" as he described the experience:

"But dear brother, think, further think for a moment, what joy filled our hearts, and with surprise we must have bowed (for who should not have bowed the knee for such a blessing!) when we received under his hands the Holy Priesthood as he said, 'Upon you my fellow-servants, in the name of Messiah, I confer this Priesthood and this authority, which shall remain upon earth "that" the sons of Levi may yet offer an offering unto the Lord in righteousness." (Messenger and Advocate, p.16.)

So considering other scriptures and Cowdery's report, the word "until" in D&C 13:1, should be understood to mean that the Aaronic priesthood would be on the earth "so that" the sons of Levi could use it to make their sacrifice.

According to Joseph Fielding Smith:
"The Priesthood of Aaron, or the Levitical Priesthood, will not end when the sons of Levi make their offering in righteousness, but it will remain on the earth as long as mortals dwell here. . . . As long as we have temporal things on the earth this priesthood is necessary. Eventually, when the earth is celestialized, I suppose all priesthood will be of the higher order." (Smith, Doctrines of Salvation 3:91-92.)