Maimonides explains (Laws of Prayer, 1:4): "Once the people of Israel went into exile in the days of Nebuchadnezzar the wicked, they mingled with Persia and Greece and other peoples, and children were born to them in the lands of these other peoples, and these children confused their languages, so that their tongues were mixed with other languages and you could not express their needs in a single language. […] Therefore, when one of them prayed, he could not ask for his desires or express the praises of God in the Holy Tongue. […] When Ezra and his Court saw this, they established eighteen ordained blessings.".
This is merely the introduction to a lengthy and detailed explanation of the source of prayer, its various parts, and its order. Just as there is a protocol for greeting, conversing, and dining with a flesh-and-blood king, there is a protocol for communicating with God, the King of all Kings. Beginning on the second day of Passover, several changes are introduced to this protocol, and to avoid forgetting them, here is a brief list of modifications.
1. In the Amidah (the standing prayer) it began to be said "He makes the dew descend" in the second blessing.
2. In the Blessing of the Years, it began to be said "And give blessing" (instead of "And give dew and rain").
3. During the days of Passover, a request is added to be remembered at the festival for kindness, blessing, and a good life (in Hebrew, "iaale veIavó", 'may it rise and come').
4. Half "Hallel" is recited during the rest of the days of Passover.
5. The "Musaf" prayer is recited, saying the part corresponding to the last days of Pesach.
6. Tefillin are not worn during the days of Passover.
Thank you, Rabbi.