The present day is influenced by the days which preceded it, Shabbos Chazak, and erev Shabbos, the 15th of Teves, "the day the moon shines in its fullness." The latter represents a fullness of all the spiritual service connected with the month of Teves, the month "when the body derives pleasure from the body."
Since this quality was expressed on erev Shabbos, it also influenced the Shabbos as implied by our Sages' expression, "Whoever prepares on erev Shabbos, eats on Shabbos." "From the Shabbos are blessed all the coming days;" accordingly, this quality also effects the coming days, in particular today, Sunday, the 17th of Teves. 17 is numerically equivalent to Tov -- "good." Thus, it represents "the good of Teves." Similarly, tonight begins the 18th of Teves. 18 is numerically equivalent to chai, "life." Thus, it represents "the life of Teves," which as explained above is distinguished as "the month in which the body derives pleasure from the body."
The Torah refers to Sunday as "one day" rather than "the first day." Our Sages explain that this expression alludes to the fact that He was "at one with His world." Though the entire creation came into being on the first day, nevertheless, G-d was "at one" with the creation in its entirety.
Each week, the entire cycle of time begins again. Though many thousands of weeks have passed since the first week of creation, Shabbos takes us above the cycle of time. Therefore, each week, the cycle repeats itself. However, based on the principle, "Always proceed higher in holy matters," each week, the cycle is repeated on a higher plane.
The connection of Sunday with the oneness of G-d and creation is also emphasized by the Psalm of the day, "the Psalm which the Levi'im would recite in the Bais HaMikdash." That Psalm begins, "The earth and all therein is the L-rd's." This is particularly true in the present age when we are approaching the Messianic redemption, the era when G-d's oneness will be openly revealed.
Until then, the Jews are given the obligation to serve G-d in the exile through the Torah. When the Jews study Torah, it is revealed to them and they reveal in the world at large, the true nature of things. Thus, just as the recitation of these Psalms by the Levi'im was an aspect of service, similarly, this service must be reflected in the service of each Jew who should meditate on the oneness of G-d when reciting this Psalm each Sunday. This, in turn, will hasten the era characterized by "The Song of the Shabbos day", "the era which is only Shabbos and rest forever."
