Dora Bas Rivka Silver O'H
According to the introductory note, this Psalm of Asaf was played on the instrument called the gittis. Asaf says to sing joyfully to G-d, Who is our strength. (Here He is called the G-d of Jacob, who is the Forefather that went down to Egypt, beginning our very first exile.) Play the harp and cymbals to G-d. Blow shofar for the appearance of the moon, for the appointed holiday (that is, Rosh Hashana), which is a decree for Israel from G-d. (Verses 4-5 are the Kiddush for the day of Rosh Hashana.) G-d gave us this law as a testimony that on Rosh Hashana He freed Joseph from prison in Egypt. (Here, Joseph, normally Yosef in Hebrew, is Yehosef, with the letter Hei of G-d's Name added. We've seen this phenomenon before, such as Jonathan, which is sometimes Yonason and sometimes Yehonason - see I Samuel 18.)
G-d removed the burden of Egyptian servitude from the Jews, who cried out to Him for help. He displayed His might publicly in Egypt, saving the Jews even though He knew they would later sin (as they did in the desert when they complained). If only Israel would listen to G-d, Who gave them the Torah! He said not to turn to idols, only to serve Him. If we would obey, all our needs would be met! But the nation did not listen and cling to G-d. He let them stray, come what may, though He'd prefer they listen to His words and follow His paths. If they did, He'd conquer their foes for them.
G-d's enemies deny their evil deeds, but they will get the fates they deserve. When Israel follows Him, G-d will feed them "cream of wheat," just as He cared for their needs in the desert. ("Cream of wheat" is not a breakfast food, but an expression meaning "the best of...")
Psalm 81 is the Psalm of the day for Thursday. Fish and birds were created on Thursday and people praise G-d in awe when they see the incredible diversity of His creations. (See Talmud Rosh Hashana 31a.)