Parshas Bereishis

The shir of Shabbos begins with an introduction מִזְמוֹר שִׁיר לְיוֹם הַשַּׁבָּֽת this is the shir of Shabbos. (Point to ponder: why doesn't any other shir shel yom start with the same introduction?) If you read and learn the rest of the chapter you will not find a single reference to Shabbos. How did this chapter become the chapter of Shabbos? Why when you say this chapter, do some poskim hold that you were mekabel Shabbos?
Let us learn the chapter of Tehilim to get a better understanding. טוֹב לְהֹדוֹת לַיהֹוָה וּלְזַמֵּר לְשִׁמְךָ עֶלְיֽוֹן- everything Hashem does is perfect and should be praised. לְהַגִּיד בַּבֹּקֶר חַסְֽדֶּךָ - in the dawn when everything is bright, tell the world of Hashem's chesed. וֶאֱמֽוּנָֽתְךָ בַּלֵּילֽוֹת- when it's dark and there's no hope in sight, talk of your faith in Him. Dovid Hamelech goes on to answer the question that has bothered everyone since Moshe: בִּפְרֹחַ רְשָׁעִים כְּֽמוֹ עֵשֶׂב - Why do the wicked florish? Why is it that the evil can enjoy the world while the rightous suffer? To this King Dovid answers: לְהִשָּֽׁמְדָם עֲדֵי־עַֽד - they will be lost forever. The wicked enjoy this world for it is all they have, they don't have a part in the world to come. צַדִּיק כַּתָּמָר יִפְרָח כְּאֶרֶז בַּלְּבָנוֹן יִשְׂגֶּֽה- The righteous don't need the pleasures of this world. שְׁתוּלִים בְּבֵית יְהֹוָה בְּחַצְרוֹת אֱלֹהֵינוּ יַפְרִֽיחוּ - they will florish and enjoy the house of Hashem in the world to come. The theme of the mizmor is perfection. If you look at the world through Hashem's lens you will not see any bad in the world- every good deed is payed in full, while no bad deed will go unaccounted for. We just dont have the eyes to see it and sometimes we need to rely on our faith, לְהַגִּיד כִּֽי־יָשָׁר יְהֹוָה צוּרִי וְֽלֹא עַוְלָתָה בּֽוֹ - that all of Hashems ways are perfect.
The final posuk of the six days of creation- the posuk that ushered in the first Shabbos is: וַיַּרְא אֱלֹהִים אֶת-כָּל-אֲשֶׁר עָשָׂה, וְהִנֵּה-טוֹב מְאֹד - Hashem looked at the entire creation, at everthing that will ever traspire in the world- the yetzar tov and the yetzar horah, the good and the evil, everything- and it was very good- perfect. וַיְהִי-עֶרֶב וַיְהִי-בֹקֶר, יוֹם הַשִּׁשִּׁי - with that knowledge the world was ready to enter Shabbos. Shabbos is 'Yom shekulo tov' the day when all in the world is perfect, a day that we leave all of our worries of the week and we enter the world of Hashem- the world of perfection.
What is a more appropriate shir to welcome the Shabbos then the shir that sings of the perfection of the world?

Adopted from Pachad Yitzchok- Shabbos mamer 2.