It is difficult to feel two contradictory emotions at the same time. Conflicting, if not contradictory, emotions such as joy and sadness, love and hate, fear and comfort, do not easily co-exist. As human beings constantly struggle with conflicting emotions, Judaism, wanting to give each its proper time for expression, separates differing emotions into separate days.
The fear that gripped the Jewish people during the time of Haman is remembered on the 13th of Adar, with the 14th (or 15th) reserved for celebration. The festive mood that accompanied the second receiving of the Torah and the granting of forgiveness—events that occurred on Yom Kippur—is celebrated on the 9th of Tishrei, when we are mandated to eat a festive meal; whereas the 10th of Tishrei is a day of serious, even fearful introspection. Similarly, in modern times, the too-many tragedies endured by the state of Israel are commemorated on the 4th of Iyar; the 5th is a day of celebration and thanksgiving to G-d for this wonderful opportunity to build a Jewish state.
One would certainly not expect the emotions of fear and joy to go hand in hand. And as a rule, they do not. Where there is great joy, fear has no place; and when one is afraid, there is little rejoicing. The Torah understands this to be true on a human level. But when dealing with the Divine, joy and fear are meant to co-exist. Sadly, we often have to fear man, but fear of G-d is meant to be awe-inspiring and ultimately evoke feelings of great joy.
“You shall eat before the G-d, your Lord in the place that he will choose as dedicated to His name the second tithe of your grain, wine, and oil, so that you will learn to fear G-d for all times" (Devarim 14:23). Four years out of seven, the Jewish farmer was to bring ma’aser sheni, the second tithe, consisting of just under 10% of his produce (or if one preferred, 125% of the cash equivalent) to Jerusalem and eat (or spend the proceeds of) it there. This eating was meant to instill fear of G-d in the Israeli farmer.
Being in the presence of G-d is the key to fearing G-d. When one comes "face to face" with “the great, mighty, and awesome G-d” (Devarim 10:17)—and we all will, sooner or later—how can one not be full of fear? As so the farmer comes to the place "where you see the Divine presence, the Kohanim in their service, the Levites on their platforms and Israelites in their prayer assemblies" (Rashbam) and "the place of the High Court that teaches and issuing ruling" (Seforno) to learn to "fear G-d" .
Such feelings are not easy to come by. As he lay on his deathbed surrounded by his students, the future leaders of the Jewish people, Rav Yochanan ben Zachai blessed them that they should fear G-d as much as they did man. When questioned about this rather obvious and understated demand, he answered, "If only" (Brachot 28b). If only we would truly fear G-d, what a different world this would be! Fear of G-d must motivate us to higher and higher spiritual levels, bringing us greater and greater happiness.
At the same time being in the presence of G-d is the key to great rejoicing. And a scant three verses later, the Torah tells us, "Eat it there before your Lord, so that you and your family will be able to rejoice". It is not easy to feel the divine presence even for those who punctiliously perform the mitzvot of the Torah. But for those who do, it is a joy unmatched. Imagine the feelings, lehavdil, of an artist meeting Picasso, a scientist meeting Einstein, or a musician Mozart. How much greater is that feeling of joy when "meeting" G-d and feeling His caring hand.
This joy is reflected in our mitzvot. The lulav was taken up for one day outside the Temple, but for seven days in the Temple precincts. "And you shall rejoice before G-d for seven days" (Vayikra 23:40). We rejoice when we are before G-d, and there is no place closer to G-d than the Temple. Yom Kippur, the day we “purify ourselves before G-d”, is also the happiest day of the year. Feeling the Divine presence and knowing that G-d is the ultimate ruler of this world brings great joy. And the parsha ends with the mitzva of simcha on Yom Tov, the days we are obligated to travel to the Beit HaMikdash.
To feel the power the majesty the grandeur of the Creator—to be awestruck by His creation—is a feeling of great fear and joy.
The Jewish farmer must come to Jerusalem, the place where the Divine presence rests, to be inspired, to be "frightened", and to experience joy. May the day speedily arrive when the only fear in Jerusalem will be that of G-d, bringing with it joy for all.
 The Rambam actually defines “before G-d” to include Jerusalem and thus, rules that one fulfills a Biblical command all seven days when taking the arba minim in Jerusalem.