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
psa 133:1The blessings of fraternal unity. (Psa 133:1-3)
As the fragrant oil is refreshing, so this affords delight. The holy anointing oil for the high priest was olive oil mixed with four of the best spices (Exo 30:22, Exo 30:25, Exo 30:30). Its rich profusion typified the abundance of the Spirit's graces. As the copious dew, such as fell on Hermon, falls in fertilizing power on the mountains of Zion, so this unity is fruitful in good works.
psa 133:3there--that is, in Zion, the Church; the material Zion, blessed with enriching dews, suggests this allusion the source of the influence enjoyed by the spiritual Zion.
commanded the blessing--(Compare Psa 68:28).
stand in the house of the Lord--at the time of the evening sacrifice, to unite in praising God in their name and that of the people, using appropriate gestures, to which the priests reply, pronouncing the Mosaic blessing which they alone could pronounce. A fit epilogue to the whole pilgrim-book, Psalms 120-134.
by night--the evening service (Psa 141:2), as opposed to morning (Psa 92:2).