John Lightfoot on Matthew 21:9

[Hosanna to the Son of David.] Some are at a loss why it is said to the Son, and not O Son: wherefore they fly to Caninius as to an oracle, who tells us, that those very bundles of boughs are called Hosanna; and that these words, Hosanna to the Son of David, signify no more than boughs to the Son of David. We will not deny that bundles are sometimes so called, as seems in these clauses...where it is plain, that a branch of palm is called Lulab, and boughs of myrtle and willow bound together are called Hosanna: but, indeed, if Hosanna to the Son of David signifies boughs to the Son of David, what do those words mean, Hosanna in the highest? The words therefore here sung import as much as if it were said, We now sing Hosanna to the Messias.

In the feast of Tabernacles, the great Hallel, as they call it, used to be sung, that is, Psalm 113-118. And while the words of the Psalms were sung or said by one, the whole company used sometimes to answer at certain clauses, Halleluia. Sometimes the same clauses that had been sung or said were again repeated by the company: sometimes the bundles of boughs were brandished or shaken. "But when were the bundles shaken?" The rubric of the Talmud saith, "At that clause Give thanks unto the Lord, in the beginning of Psalm 118, and at the end. And at that clause, Save now, I beseech thee, O Lord, (Psa 118:25) as saith the school of Hillel: but the school of Shammai saith also, at that clause, O Lord, I beseech thee, send now prosperity. R. Akibah said, I saw R. Gamaliel and R. Joshuah, when all the company shook their bundles they did not shake theirs, but only at that clause, Save now, I beseech thee, O Lord."

On every day of the feast, they used once to go round the altar with bundles in their hands, singing this, Save now, I beseech thee, O Lord; I beseech thee, O Lord, send now prosperity. But on the seventh day of the feast they went seven times round the altar, &c. "The tossing or shaking of the bundles was on the right hand, on the left hand, upwards and downwards."

"The reason of the bundles was this, because it is written, 'Then let all the trees of the wood sing,' (Psa 96:12). And afterward it is written, 'Give thanks unto the Lord, because he is good,' (Psa 106:1). And afterward, 'Save us, O Lord, O our God,' &c. (Psa 106:47). And the reason is mystical. In the beginning of the year, Israel and the nations of the world go forth to judgment; and being ignorant who are to be cleared and who guilty, the holy and blessed God commanded Israel that they should rejoice with these bundles, as a man rejoiceth who goeth out of the presence of his judge acquitted. Behold, therefore, what is written, 'Let the trees of the wood sing'; as if it were said, Let them sing with the trees of the wood, when they go out justified from the presence of the Lord," &c.

[For more information on feast days, please see "The Temple: Its Ministry and Services" by Alfred Edersheim.]

These things being premised concerning the rites and customs of that feast, we now return to our story:--

I. It is very much worth our observation, that the company receives Christ coming now to the Passover with the solemnity of the feast of Tabernacles. For what hath this to do with the time of the Passover? If one search into the reason of the thing more accurately, these things occur; First, The mirth of that feast above all others; concerning which there needs not much to be said, since the very name of the feast (for by way of emphasis it was called Festivity or Mirth) sufficiently proves it. Secondly, That prophecy of Zechariah, which, however it be not to be understood according to the letter, yet from thence may sufficiently be gathered the singular solemnity and joy of that feast above all others; and, perhaps, from that same prophecy, the occasion of this present action was taken. For being willing to receive the Messias with all joyfulness, triumph, and affection of mind (for by calling him the Son of David, it is plain they took him for the Messias), they had no way to express a more ardent zeal and joy at his coming, than by the solemn procession of that feast. They have the Messias before their eyes; they expect great things from him; and are therefore transported with excess of joy at his coming.

II. But whereas the Great Hallel, according to the custom, was not now sung, by reason of the suddenness of the present action, the whole solemnity of that song was, as it were, swallowed up in the frequent crying out and echoing back of Hosanna; as they used to do in the Temple, while they went round the altar. And one while they sing Hosanna to the Son of David; another while, Hosanna in the highest; as if they had said, "Now we sing Hosanna to the Son of David; save us, we beseech thee, O thou [who dwellest] in the highest, save us by the Messias."