Rashi: It discusses the world to come, which is totally Shabbos (Rosh Hashanah 31a).

Radak: It discusses the days of Mashi'ach, which are totally Shabbos (Rosh Hashanah 31a).

Malbim: It discusses Hashem's Hashgachah in His world. It looks like everything is based on nature, but one who is wise will see His Hashgachah - punishment of Resha'im and reward of Tzadikim. Hashem hid His Hashgachah from the masses, to leave room for choice.

Refer to 90:1:2:1.

Radak citing Shocher Tov: Adam ha'Rishon said it. Refer to 92:2:1:1.

Rashi: The Leviyim said it [in the Mikdash] on Shabbosos. Also refer to 92:1:1:1.

Malbim: Shabbos is the testimony of the conduct of Hashgachah; the world is not left to nature and chance. Therefore, this Mizmor (which discusses Hashgachah) was established for Shabbos.