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 118:1After invoking others to unite in praise, the writer celebrates God's protecting and delivering care towards him, and then represents himself and the people of God as entering the sanctuary and uniting in solemn praise, with prayer for a continued blessing. Whether composed by David on his accession to power, or by some later writer in memory of the restoration from Babylon, its tone is joyful and trusting, and, in describing the fortune and destiny of the Jewish Church and its visible head, it is typically prophetical of the Christian Church and her greater and invisible Head. (Psa. 118:1-29)
The trine repetitions are emphatic (compare Psa 118:10-12, Psa 118:15-16; Psa 115:12-13).
Let . . . say--Oh! that Israel may say.
now--as in Psa 115:2; so in Psa 118:3-4. After "now say" supply "give thanks."
that his mercy--or, "for His mercy."
psa 118:5distress--literally, "straits," to which "large place" corresponds, as in Psa 4:1; Psa 31:8.
psa 118:6Men are helpless to hurt him, if God be with him (Psa 56:9), and, if enemies, they will be vanquished (Psa 54:7).
psa 118:8Even the most powerful men are less to be trusted than God.
psa 118:10Though as numerous and irritating as bees [Psa 118:12], by God's help his enemies would be destroyed.
psa 118:12as the fire of thorns--suddenly.
in the name, &c.--by the power (Psa 20:5; Psa 124:8).
psa 118:13The enemy is triumphantly addressed as if present.
psa 118:15rejoicing and salvation--the latter as cause of the former.
psa 118:16right hand . . . is exalted--His power greatly exerted.
psa 118:17He would live, because confident his life would be for God's glory.
psa 118:19Whether an actual or figurative entrance into God's house be meant, the purpose of solemn praise is intimated, in which only the righteous would or could engage.
psa 118:22These words are applied by Christ (Mat 21:42) to Himself, as the foundation of the Church (compare Act 4:11; Eph 2:20; Pe1 2:4, Pe1 2:7). It may here denote God's wondrous exaltation to power and influence of him whom the rulers of the nation despised. Whether (see on Psa 118:1) David or Zerubbabel (compare Hag 2:2; Zac 4:7-10) be primarily meant, there is here typically represented God's more wonderful doings in exalting Christ, crucified as an impostor, to be the Prince and Saviour and Head of His Church.
psa 118:24This is the day--or period distinguished by God's favor of all others.
psa 118:25Save now--Hebrew, "Hosanna" (compare Psa 115:2, &c., as to now) a form of prayer (Psa 20:9), since, in our use, of praise.
psa 118:26he that cometh . . . Lord--As above intimated, this may be applied to the visible head of the Jewish Church entering the sanctuary, as leading the procession; typically it belongs to Him of whom the phrase became an epithet (Mal 3:1; Mat 21:9).
psa 118:27showed us light--or favor (Psa 27:1; Psa 97:11). With the sacrificial victim brought bound to the altar is united the more spiritual offering of praise (Psa 50:14, Psa 50:23), expressed in the terms with which the Psalm opened.