Psalms 92:1
מִזְמוֹר שִׁיר לְיוֹם הַשַּׁבָּת
A Psalm, a song for the Sabbath day.
It’s easy to see why Psalm 92 was selected as the Psalm of the Day for Shabbos – it says so right in the first verse! But the Talmud (Rosh Hashana 31a) clarifies that this doesn’t only refer to the day of rest that comes at the end of each week. It also refers to the future messianic era, which is called “a day that is completely Shabbos.”