Every night we say two berachos after reading the Shema. The second beracha concludes: "Blessed are You Hashem who guards His People Yisrael for ever." On the Shabbos however the text is changed to: "and spread over us the shelter of Your peace. Blessed are You Hashem, Who spreads the shelter of peace over us, over all Your people Yisrael and over Yerushalaim." The Taz explains that since the day of Shabbos itself brings protection of Yisrael there is no need to pray for protection on Shabbos night.

It is for this reason that it is forbidden to erect a (makeshift) tent (for a protection) on Shabbos, because by doing so one separates oneself from the Divine 'tent' which Hashem Himself erects to contain all Yisrael on this day.