Biblical Commentary on the Old Testament, by Carl Friedrich Keil and Franz Delitzsh, [1857-78], at sacred-texts.com
In this Psalm, says Hengstenberg, "David brings to the consciousness of the church the glory of the fellowship of the saints, that had so long been wanting, the restoration of which had begun with the setting up of the Ark in Zion." The Psalm, in fact, does not speak of the termination of the dispersion, but of the uniting of the people of all parts of the land for the purpose of divine worship in the one place of the sanctuary; and, as in the case of Psa 122:1-9, its counterpart, occasions can be found in the history of David adapted to the לדוד of the inscription. But the language witnesses against David; for the construction of שׁ with the participle, as שׁיּרד, qui descendit (cf. Psa 135:2, שׁעמדים, qui stant), is unknown in the usage of the language prior to the Exile. Moreover the inscription לדוד is wanting in the lxx Cod. Vat. and the Targum; and the Psalm may only have been so inscribed because it entirely breathes David's spirit, and is as though it had sprung out of his love for Jonathan.
With גּם the assertion passes on from the community of nature and sentiment which the word "brethren" expresses to the outward active manifestation and realization that correspond to it: good and delightful (Psa 135:3) it is when brethren united by blood and heart also (corresponding to this their brotherly nature) dwell together - a blessed joy which Israel has enjoyed during the three great Feasts, although only for a brief period (vid., Psa 122:1-9). Because the high priest, in whom the priestly mediatorial office culminates, is the chief personage in the celebration of the feast, the nature and value of that local reunion is first of all expressed by a metaphor taken from him. שׁמן הטּוב is the oil for anointing described in Exo 30:22-33, which consisted of a mixture of oil and aromatic spices strictly forbidden to be used in common life. The sons of Aaron were only sprinkled with this anointing oil; but Aaron was expressly anointed with it, inasmuch as Moses poured it upon his head; hence he is called par excellence "the anointed priest" (הכּהן המּשׁיה), whilst the other priests are only "anointed" (משׁחים, Num 3:3) in so far as their garments, like Aaron's, were also sprinkled with the oil (together with the blood of the ram of consecration), Lev 8:12, Lev 8:30. In the time of the second Temple, to which the holy oil of anointing was wanting, the installation into the office of high priest took place by his being invested in the pontifical robes. The poet, however, when he calls the high priest as such Aaron, has the high-priesthood in all the fulness of its divine consecration (Lev 21:10) before his eyes. Two drops of the holy oil of anointing, says a Haggada, remained for ever hanging on the beard of Aaron like two pearls, as an emblem of atonement and of peace. In the act of the anointing itself the precious oil freely poured out ran gently down upon his beard, which in accordance with Lev 21:5 was unshortened.
In that part of the Tra which describes the robe of the high priest, שׁוּלי is its hems, פּי ראשׁו, or even absolutely פּה, the opening for the head, or the collar, by means of which the sleeveless garment was put on, and שׂפה the binding, the embroidery, the border of this collar (vid., Exo 28:32; Exo 39:23; cf. Job 30:18, פּי כתנתּי, the collar of my shirt). פּי must apparently be understood according to these passages of the Tra, as also the appellation מדּות (only here for מדּים, מּדּים), beginning with Lev 6:3, denotes the whole vestment of the high priest, yet without more exact distinction. But the Targum translates פּי with אמרא (ora = fimbria) - a word which is related to אמּרא, agnus, like ᾤα to ὄΐς. This ᾤα is used both of the upper and lower edge of a garment. Accordingly Appolinaris and the Latin versions understand the ἐπὶ τὴν ὤαν of the lxx of the hem (in oram vestimenti); Theodoret, on the other hand, understands it to mean the upper edging: ὤαν ἐκάλεσεν ὃ καλοῦμεν περιτραχήλιον, τοῦτο δὲ καὶ ὁ Ἀκύλας στόμα ἐνδυμάτων εἴρηκε. So also De Sacy: sur le bord de son vtement, c'est--dire, sur le haut de ses habits pontificaux. The decision of the question depends upon the aim of this and the following figure in Psa 133:3. If we compare the two figures, we find that the point of the comparison is the uniting power of brotherly feeling, as that which unites in heart and soul those who are most distant from one another locally, and also brings them together in outward circumstance. If this is the point of the comparison, then Aaron's beard and the hem of his garments stand just as diametrically opposed to one another as the dew of Hermon and the mountains of Zion. פּי is not the collar above, which gives no advance, much less the antithesis of two extremes, but the hem at the bottom (cf. שׂפה, Exo 26:4, of the edge of a curtain). It is also clear that שׁיּרד cannot now refer to the beard of Aaron, either as flowing down over the upper border of his robe, or as flowing down upon its hem; it must refer to the oil, for peaceable love that brings the most widely separated together is likened to the oil. This reference is also more appropriate to the style of the onward movement of the gradual Psalms, and is confirmed by Psa 133:3, where it refers to the dew, which takes the place of the oil in the other metaphor. When brethren united in harmonious love also meet together in one place, as is the case in Israel at the great Feasts, it is as when the holy, precious chrism, breathing forth the blended odour of many spices, upon the head of Aaron trickles down upon his beard, and from thence to the extreme end of his vestment. It becomes thoroughly perceptible, and also outwardly visible, that Israel, far and near, is pervaded by one spirit and bound together in unity of spirit.
This uniting spirit of brotherly love is now symbolised also by the dew of Hermon, which descends in drops upon the mountains of Zion. "What we read in the 133rd Psalm of the dew of Hermon descending upon the mountains of Zion," says Van de Velde in his Travels (Bd. i. S. 97), "is now become quite clear to me. Here, as I sat at the foot of Hermon, I understood how the water-drops which rose from its forest-mantled heights, and out of the highest ravines, which are filled the whole year round with snow, after the sun's rays have attenuated them and moistened the atmosphere with them, descend at evening-time as a heavy dew upon the lower mountains which lie round about as its spurs. One ought to have seen Hermon with its white-golden crown glistening aloft in the blue sky, in order to be able rightly to understand the figure. Nowhere in the whole country is so heavy a dew perceptible as in the districts near to Hermon." To this dew the poet likens brotherly love. This is as the dew of Hermon: of such pristine freshness and thus refreshing, possessing such pristine power and thus quickening, thus born from above (Psa 110:3), and in fact like the dew of Hermon which comes down upon the mountains of Zion - a feature in the picture which is taken from the natural reality; for an abundant dew, when warm days have preceded, might very well be diverted to Jerusalem by the operation of the cold current of air sweeping down from the north over Hermon. We know, indeed, from our own experience how far off a cold air coming from the Alps is perceptible and produces its effects. The figure of the poet is therefore as true to nature as it is beautiful. When brethren bound together in love also meet together in one place, and in fact when brethren out of the north unite with brethren in the south in Jerusalem, the city which is the mother of all, at the great Feasts, it is as when the dew of Mount Hermon, which is covered with deep, almost eternal snow,
(Note: A Haraunitish poem in Wetzstein's Lieder-Sammlungen begins: Arab. - - 'l-bâriḥat habbat ‛lynâ šarârt mn ‛âliya 'l-ṯlj, "Yesterday there blew across to me a spark
descends upon the bare, unfruitful - and therefore longing for such quickening - mountains round about Zion. In Jerusalem must love and all that is good meet. For there (שׁם as in Psa 132:17) hath Jahve commanded (צוּה as in Lev 25:21, cf. Psa 42:9; Psa 68:29) the blessing, i.e., there allotted to the blessing its rendezvous and its place of issue. את־הבּרכה is appositionally explained by חיּים: life is the substance and goal of the blessing, the possession of all possessions, the blessing of all blessings. The closing words עד־העולם (cf. Psa 28:9) belong to צוּה: such is God's inviolable, ever-enduring order.