A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
num 8:1HOW THE LAMPS ARE TO BE LIGHTED. (Num 8:1-4)
the Lord spake unto Moses--The order of this chapter suggests the idea that the following instructions were given to Moses while he was within the tabernacle of the congregation, after the princes had completed their offering. But from the tenor of the instructions, it is more likely that they were given immediately after the Levites had been given to the priests (see on Num. 3:1-4:49), and that the record of these instructions had been postponed till the narrative of other transactions in the camp had been made [PATRICK].
num 8:2Speak unto Aaron, &c.--The candlestick, which was made of one solid, massive piece of pure gold, with six lamps supported on as many branches, a seventh in the center surmounting the shaft itself (Exo 25:31; Exo 37:17), and completed according to the pattern shown in the mount, was now to be lighted, when the other things in the sanctuary began to be applied to religious service. It was Aaron's personal duty, as the servant of God, to light His house, which, being without windows, required the aid of lights (Pe2 1:19). And the course he was ordered to follow was first to light the middle lamp from the altar-fire, and then the other lamps from each other--a course symbolical of all the light of heavenly truth being derived from Christ, and diffused by His ministers throughout the world (Rev 4:5).
the seven lamps shall give light over against the candlestick--The candlestick stood close to the boards of the sanctuary, on the south side, in full view of the table of showbread on the north (Exo 26:35), having one set of its lamps turned towards the east, and another towards the west; so that all parts of the tabernacle were thus lighted up.
num 8:6THE CONSECRATION OF THE LEVITES. (Num. 8:5-22)
Take the Levites . . . and cleanse them--This passage describes the consecration of the Levites. Although the tribe was to be devoted to the divine service, their hereditary descent alone was not a sufficient qualification for entering on the duties of the sacred office. They were to be set apart by a special ceremony, which, however, was much simpler than that appointed for the priests; neither washing nor anointing, nor investiture with official robes, was necessary. Their purification consisted, along with the offering of the requisite sacrifices (Lev 1:4; Lev 3:2; Lev 4:4), in being sprinkled by water mixed with the ashes of a red heifer (Num 19:9), and shaved all over, and their clothes washed--a combination of symbolical acts which was intended to remind them of the mortification of carnal and worldly desires, and the maintenance of that purity in heart and life which became the servants of God.
num 8:9thou shalt gather the whole assembly of the children of Israel together, &c.--As it was plainly impossible that the whole multitude of the Israelites could do this, a select portion of them must be meant. This party, who laid their hands upon the Levites, are supposed by some to have been the first-born, who by that act, transferred their peculiar privilege of acting as God's ministers to the Levitical tribe; and by others, to have been the princes, who thus blessed them. It appears, from this passage, that the imposition of hands was a ceremony used in consecrating persons to holy offices in the ancient, as, from the example of our Lord and His apostles, it has been perpetuated in the Christian Church.
num 8:11And Aaron shall offer the Levites--Hebrew, "as a wave offering"; and it has been thought probable that the high priest, in bringing the Levites one by one to the altar, directed them to make some simple movements of their persons, analogous to what was done at the presentation of the wave offerings before the Lord. Thus were they first devoted as an offering to God, and by Him surrendered to the priests to be employed in His service. The consecration ceremonial was repeated in the case of every Levite who was taken (as was done at a later period) to assist the priests in the tabernacle and temple. (See on Ch2 29:34).
num 8:14and the Levites shall be mine--that is, exempt from all military duty or secular work--free from all pecuniary imposition and wholly devoted to the custody and service of the sanctuary.
num 8:15after that, shall the Levites go in to do the service of the tabernacle of the congregation--into the court, to assist the priests; and at removal into the tabernacle--that is, into the door of it--to receive the covered furniture.
num 8:19to make an atonement for the children of Israel, &c.--to aid the priests in that expiatory work; or, as the words may be rendered, "to make redemption for" the Levites being exchanged or substituted for the first-born for this important end, that there might be a sanctified body of men appointed to guard the sanctuary, and the people not allowed to approach or presumptuously meddle with holy things, which would expose them to the angry judgments of Heaven.
num 8:24from twenty and five years old, &c.--(Compare Num 4:3). They entered on their work in their twenty-fifth year, as pupils and probationers, under the superintendence and direction of their senior brethren; and at thirty they were admitted to the full discharge of their official functions.
num 8:25from the age of fifty years they shall cease waiting upon the service thereof, &c.--that is, on the laborious and exhausting parts of their work.
num 8:26But shall minister with their brethren--in the performance of easier and higher duties, instructing and directing the young, or superintending important trusts. "They also serve who only wait" [MILTON].