This is the last Shabbos of the month of Elul, the last month of the year. The last Shabbos of each month is called Shabbos Mevorchim, when we recite a special blessing that the coming new month will bring goodness, happiness, and peace.
Although this Shabbos is Shabbos Mevorchim, we will not recite this blessing. HaShem Himself blesses the month of Tishrei, and His blessing gives us the power to bless all of the other months of the coming year.
What blessing does HaShem give? Atem nitzavim HaYom, "Today you are standing."
What does the Torah mean by "today"? When is today? Every day? Any day? Our Rabbis tells us that the Torah is speaking about Rosh HaShanah, when we stand before HaShem in judgment.
Can you imagine how people in court look when they are being judged? When the judge pronounces a person guilty, the accused man sits hunched over in his place, with downcast eyes and a bowed head. But when the judge announces, "Not guilty," the defendant stands upright, holding his head up high, his face beaming with joy.
The Torah tells us: "Today you are standing." On Rosh HaShanah, as we all pass before HaShem for judgment, we are blessed to stand upright as our verdict is announced.
The Torah continues: "Kulchem, all of you together." When HaShem sees the Jews come together, it's like a father seeing all of his children playing joyfully together. Nothing could make Him any happier.
When HaShem sees us come together with ahavas Yisrael, He blesses us with a kesivah vachasimah tovah for a good year.
(Adapted from Sichos Parshas Nitzavim-Vayeilech, 5740)