The Levites were the master singers of the Temple. They sang psalms and played music to accompany the divine service. Every morning and afternoon, during the wine libation of the daily tamid offering, the Levites sang a specific psalm depending on the day of the week. Each daily psalm, Shir shel Yom, contains a reference to that day of Creation. For example, on Sunday, the Shir shel Yom is Psalm 24, which opens with “The earth is the Lord’s and all that it holds.” On the first day, God created heaven and earth and is therefore its master.
Today, post-Temple, daily shacharit morning services conclude with Shir shel Yom. We preface the psalm with “Today is the first (or second third, etc) day since Shabbat on which the Levites sang.” Reciting psalms just as the Levites did reminds us that prayer should be a celebration.
