Hiyya, the son of Rab, of Difti learned: ‘And you shall afflict your souls in the ninth [day of the month]’. But is one fasting on the ninth? Do we not fast on the tenth? Rather, it comes to indicate that, if one eats and drinks on the ninth, Scripture accounts it to him as if he had fasted on the ninth and the tenth.'
The Netziv (in his commentary to the שאילתות) interprets Hiyya's remarks to mean that Erev Yom Kippur merges with Yom Kippur itself to form one unit. On Erev Yom Kippur, we celebrate the sublime opportunity, nay the incalculable privilege, that God has given us to spend a full day in His Presence, basking in the unmitigated light of the the Shekhina. He offers us this time of intimacy with Him to ask forgiveness for our sins, to reconcile with one another and with Him. We know, deep in our hearts, that God's endless love for us will lead to סליחה, מחילה וכפרה if we only return to Him and set ourselves on the path of Teshuvah. So, the Torah instructs us to celebrate our quality time with God, our anticipated immersion in sanctity and our hoped for forgiveness, in advance of Yom Kippur itself!!
This means that both the ninth and the tenth of Tishrei possess a unique tangible charisma, קדושת היום.
You don't really need Hiyya bar Rav to teach you this truth. All day, as I saw everyone go about their business in the Hills of Judea and in Jerusalem, you could feel the sense of anticipation. 'Yom ha-Qadosh' (as my late father in law referred to Yom Kippur, מיט א ציטער) is upon us. Already tonight, the atmosphere is infused with an aetherial, other worldly feeling. It is the Ninth of Tishrei, and the sanctity of Yom Kippur is steadily descending and intensifying. It will grow, as the country winds down, and stops starting tomorrow night.
Listen carefully. The harbingers of the שכינה are already here. They are in the profound silence outside my window. They are gathering at the Kotel, where thousands of בני עדות המזרח are gathered for their most exalted selihot. The King is coming. The Shekhina is descending. A mixture of anticipation and awe, fear and excitement fill Eretz Yisrael, which is blessed not only with Yom Kippur but with Erev Yom Kippur.
אשרי העם שככה לו. אשרי העם שד' א-לקיו
גמר חתימה טובה לכל בית ישראל