מִזְמ֥וֹר שִׁ֗יר לְי֣וֹם הַשַּׁבָּֽת׃
Psaume. Cantique pour le jour du Sabbat.
Rashi on Psalms
A song with musical accompaniment of the Levites.
Rashi on Psalms
for the Sabbath day that they would recite on Sabbaths, and it deals with the world to come, which is entirely Sabbath.