When we recite Kiddush on Shabbat evening, we recall the story of the original Shabbat - how God rested on and blessed the seventh day to make it holy. But in a seemingly odd manner, we begin the story with two unconnected words "יום הששי" "the sixth day". These words are in fact the final words of the previous chapter and the concluding description of (you guessed it) the sixth day of creation - not highly appropriate for the blessing of the 7th day, Shabbat. To understand this, let's take a step back. In creating this world, one of the main motifs of which God makes use is separation - He separates between light and darkness, between water and land, and between the 6 days of creation and Shabbat. We emulate God by following similar guidelines, living our lives with prescribes boundaries and like God, setting aside the 7th day as a day of rest, Shabbat. With one small difference: God, who knows in His infinite wisdom how to distinguish with perfect measurements between the 6th day and the 7th day, stops a hairsbreadth before Shabbat; man, in our less-than-perfect abilities cannot achieve the same precision in our distinctions.
To solve this problem, we have Tosefet Shabbat ("Adding onto Shabbat"), a commandment derived to be biblical which compensates for our inability to precisely draw the boundaries of Shabbat by adding additional time onto Shabbat, starting it a little earlier and ending it a little later. Regarding this concept, the Mei Marom asks a question: We know that God keeps Shabbat (because we learn Shabbat from God), but does God keep Tosefet Shabbat? He answers that God does not and can not keep Tosefet Shabbat, for if He were to keep it it would not be Tosefet Shabbat, but rather Shabbat itself. What emerges from this clever answer is that man actually has a unique ability entrusted to him - the ability to add on to Shabbat. And so the creation of holiness, which was previously exclusively God's domain, has now become man's Divinely-mandated responsibility as well. The commandment of Tosefet Shabbat thus becomes not merely a precaution from holiness but rather a Divine empowerment - for man to create, to transform, and to transcend the boundaries of holiness.
So on Friday evening, when we are beginning our journey of holiness and telling the story of Shabbat, we first want to remember our ability to transform the mundane world around us into holiness. And by starting with the tail-end of Friday's story, "the sixth day", that's exactly what we're doing - adding on a bit of Friday into the holiness of Shabbat.
Practically, holiness takes many forms in our lives. It can be the language we speak, the group with which we pray and study, the religious community of which we are a part. And it is important to strengthen and uphold those fortresses of holiness and keep them holy. But if we were to stop there we would be remiss, for we would be failing to actualize our greatest God-given potential - to influence the world around us. We would miss out on the opportunity to bring forth holiness beyond its boundaries, and to transcend our own boundaries of holiness. There are so many people outside the high walls of our holy community who never had the background/exposure/courage to gain entry. You have the power to extend beyond the normal limits of holiness, to welcome people into the community of holiness of which you are a part. You can share the holiness of Shabbat with people unfamiliar with Shabbat or untouched by its holiness. You have the ability and the God-given responsibility to create transcendent holiness in this world. You have the potential to be Divine.
It is likely no coincidence that the acronym of those last two words from Friday's story and the first two words of Shabbat's story ("יום הששי. ויכלו השמים") form the name of God - for that bridge into holiness, the transformation of the mundane and transcendence of the limits of holiness, that is the manifestation of Godliness in this world. And so every Friday night, when you open Kiddush with those seemingly out-of-place words, remember your God-like and God-given potential to transform the world beyond yourself and to add holiness to those around you.