The mitzvah of Shabbos first appears in Parshas Bereishis. Although technically Shabbos is only the actual seventh day of the week, there is a mitzvah to add on to Shabbos at its beginning and at its conclusion. This mitzvah, known as tosefes Shabbos, is derived by Chazal from a posuk in Parshas Emor which requires us to begin Yom Kippur early and end it late; this is extended to Shabbos as explained in Rosh Hashana 9a.
There is a fundamental debate as to the halachic nature of this mitzvah. One possibility is that this mitzvah is meant as a safeguard against Shabbos desecration. Since it is possible for human beings to err as to the time when Shabbos begins and ends, the Torah imposes this buffer to prevent a mistake which could result in chillul Shabbos. Rashi (Beraishis 2:2) takes this approach in explaining that it was unnecessary for Hashem to begin His Shabbos early because Hashem knows the precise time when Shabbos begins. Alternatively, one can understand that the Torah wants us to begin Shabbos early and end it late not merely to be cautious, but because we are supposed to infuse a small part of the week with the sanctity of Shabbos.
There are practical halachic differences between these two approaches. According to the first understanding, when we accept Shabbos early we are merely refraining from melacha. It is still halachically Friday for all purposes. We refrain from melacha at a time which is not yet Shabbos lest we violate the actual Shabbos. The second approach views the time of tosefes Shabbos as being endowed with actual kedushas Shabbos, i.e. Shabbos has begun even though the astronomical day of Friday has not ended. Tosfos (Pesachim 99b) quotes two opinions whether one can fulfill one's obligation of eating the Shabbos meal during the period of tosefes Shabbos. If this time is not yet Shabbos, although one is refraining from melacha one cannot fulfill the mitzvah of eating a Shabbos meal. If tosefes Shabbos transforms Friday into Shabbos, one can fulfill the Shabbos obligation of having a meal during this time.
These two approaches to tosefes Shabbos are not only relevant in terms of technical halacha, but each approach highlights a different dimension of Shabbos observance. The terms shamor and zachor in the Aseres Hadibros describe the dual aspects of Shabbos observance. The Ramban in Parshas Yisro comments that shamor, which is synonymous with refraining from melacha, is rooted is yiras Hashem- the fear and awe we feel in the presence of Hashem. All prohibitions in the Torah emanate from yiras Hashem. Yirah requires one to step back and not act in violation of Hashem's word. Zachor, which is synonymous with the positive mitzvos of Shabbos, is rooted in ahavas Hashem- the love of Hashem. All positive mitzvos are rooted in ahavas Hashem as we try to actively come closer to Hashem by expressing our love by doing His mitzvos.
Each of these dimensions of Shabbos is expressed during the period of tosefes Shabbos. Shamor requires us to be extra careful not to violate melacha on Shabbos, and as such begin Shabbos early lest we chas v'shalom do melacha when it is actually Shabbos. Zachor fills us with such a love of Shabbos that we are not supposed to be satisfied with kedushas Shabbos being limited to the day of Shabbos itself. Rather, we are to be eager to begin Shabbos that we actually endow part of Friday with kedushas Shabbos.
Having just completed the yomim tovim of Tishrei we have experienced both of these Shabbos themes in our celebration of yom tov. Rosh Hashanah and Yom Kippur, known as the Yomim Noraim, highlight yiras Hashem. Succos and Shemini Atzeres-Simchas Torah, known as Zman Simchasenu, highlight ahavas Hashem. Shabbos is the t'chila l'mikroei kodesh - the first of all the holy days. Shabbos sets the tone for our celebration of the yomim tovim. As we begin a new year with Shabbos Bereishis, let us focus on these two dimensions of Shabbos. The mitzva of tosefes Shabbos enables us to express both our yiras Hashem and our ahavas Hashem. Hopefully this mitzvah will impact upon our entire avodas Hashem improving both our yirah and ahava for Hashem.