It states (Bamidbar 10:10), וביום שמחתכם, "On your day of joy," and Chazal (Sifri 19) say that this refers to Shabbos.
We should be happy every day of the year, as it states (Tehillim 100:2), עבדו את ה' בשמחה, "Serve Hashem with joy," but there is an added importance to being happy on Shabbos. As it is, יום שמחתכם, "Your day of joy."
As we say in the zemiros, ביום השבת קודש שישו ושמחו כמקבלי מתן נחליאל, "Be happy and joyous on Shabbos as those who received the Torah."
Some say that the tochachah (curses of the Torah) come when people aren't happy on Shabbos. The hint is from the pasuk (Devarim 28:47), תחת אשר לא עבדת את השם אלקיך בשמחה ובטוב לבב. The Bnei Yissaschar (Igra d'Kalah) explains that תחת אשר can be translated as "under [or after] the letters אש"ר." In the order of the alef beis, the letters after אש"ר are שב"ת. So the pasuk says that the tochachah comes when you don't serve Hashem with joy on Shabbos.
It states in sefer Masuk m'Dvash (from Reb Yitzchak Parchi zt'l 6), "We saw, with our own eyes, people who had a problem when Shabbos arrived, but they took their minds off their tzarah, they were happy with the Shabbos, and didn't let their worries enter their heart, and when Shabbos ended, the problem was gone. Everything turned around to joy. They were saved from their tzaraos through wonderful miracles."
Reb Yaakov Yosef Herman zt'l was from the tzaddikim who lived in America about a hundred years ago. He set an example that one can be an ehrlicher Yid even in America. One of the mitzvos that he excelled in was hachnasas orchim. Every Shabbos, many guests sat around his table.
Once, his daughter was very ill, and Reb Yaakov Yosef and his Rebbetzin stayed with her in the hospital around the clock. As Shabbos was approaching, they decided to go home so that they could perform their weekly mitzvah of hachnasas orchim.
There was another Herman in the hospital, not related to them, and that Herman was niftar on Shabbos. According to American law, the hospital had to notify the next of kin as quickly as possible, so they sent a messenger to the Herman home. But accidentally, they confused the two Herman families, and the messenger was sent to the home of Reb Yaakov Yosef Herman.
When the messenger arrived at the door with a letter, Rabbi and Rebbetzin Herman refused to accept it. It was Shabbos and Shabbos is a day of joy. The messenger left the letter at the home of one of the Herman relatives. That relative came to the Herman home to tell the parents the terrible news, but they refused to listen to what their relative was saying.
After havdalah, a second messenger from the hospital arrived at their door. "We're so sorry," he said. "We made a terrible mistake with the last letter. Your daughter is doing fine. In fact, over the past twenty-four hours, her health has improved. There was another Herman in the hospital who passed away. We intended to send the letter to that family, but accidentally, we sent the letter to you. But don't have to worry. Your daughter is doing very well." Their joy on Shabbos healed their daughter.
Here is another person who had his salvation by being happy on Shabbos. Friday morning, after his checkup, the doctor told him, "Your suspicions were correct. You have a kidney stone. And not just one. You have three kidney stones piled one on top of the other. The bottom stone is size 6, above it is a size 4 stone, and the top stone is size 2. We'll give it a couple of weeks; maybe matters will improve. However, if it doesn't, we will need to operate."
It was hard for this yungerman to be happy on Shabbos, due to his pain and fear of surgery, and his melancholy disposition affected his family. No one was in the mood to sing zemiros, and the delicious Shabbos food tasted bland. But suddenly, the yungerman became happy. With joy, he took out a Gemara Bava Metzia and studied daf 25. His face shone as he sang the Gemara. His family watched on, not understanding how his mood changed so quickly.
After studying the Gemara a few times, he told his family, "The Gemara discusses someone who found three coins of different sizes, one on top of the other. The largest coin is at the bottom, a medium-sized coin in the middle, and a tiny coin on top. The Gemara rules that it is evident that these coins didn't fall in this manner. Someone must have placed them there intentionally. Therefore, the finder must announce his find because the formation is a siman. I remembered this Gemara and realized that my kidney stones are in the exact same formation: the largest stone at the bottom, the medium stone in the middle, and a small stone on top. As the Gemara explains, this formation means that the stones didn't get there by chance. Obviously, they were placed there purposely by Hashem. And since it is from Hashem, I know that it is for my good." A week later, the stones left his body naturally. The joy of Shabbos brought him a remedy.
The renowned gabbai tzedakah Reb Dovid Leib Shwartz zt'l was meticulous with keeping Shabbos properly. Even in the concentration camp during the War, he kept Shabbos as best as possible. One week he told his friends that he wanted to eat three meals on Shabbos. His friends laughed at him because all they had to eat was a small piece of bread in the morning and nothing else. But he didn't let their teasing prevent him from hoping and aspiring to eat three meals on Shabbos.
Erev Shabbos, the commander of their barrack, gave everyone a loaf of bread and explained, "The bread isn't for today. Sunday, we will be trekking for three days, and this is all you will have for three days, so save the bread for then." Reb Dovid Leib didn't heed the warning. This was his opportunity. Friday night, he made Kiddush on the bread and ate some. He ate another piece in the morning and finished it at shalashudos. He didn't know how he would walk three days without food, but in the meanwhile, he was happy that he ate three meals of Shabbos. After Shabbos, their commander told them there was a change of plans. "Everyone must return the bread. We wouldn't be traveling as originally planned." Everyone returned the loaf of bread, except for Reb Dovid Leib, who had already eaten it l'kovod Shabbos.
Footnotes
10. A child was choking on a coin and his face was turning blue. The Chazon Ish zt'l exclaimed "The only solution is to bring the child to the Ponovitzer Rav, because the Ponovitzer Rav is an expert in getting coins out of people, even out of the tightest places…" Everyone laughed, including the child. This dislodged the coin and it made its way out. The Chazon Ish said about this incident, "Happiness saves people from all tzaros." We'll add that the boy wasn’t really happy. He was laughing because everyone else was laughing. But that was also enough to save him. When a person in unable to be truly happy, he should pretend to be happy, because that will also bring yeshuos.
The Yismach Yisrael zt'l said that a hint to this is in the words (Devarim 16:16) והיית אך שמח. The first letters of the three words spell שוא, falsehood, hinting that a person should also work on false joy because false joy will eventually become true joy.
There was a Slonimer Chasid who lived in Teveria called Reb Mordechai (Motke) Leider zt'l. From time to time, Reb Motke undertook the very long trip to Slonim, Lithuania, to be by his Rebbe, the Yesod HaAvodah zt’l. Reb Shmuel Salant zt'l, the Rav of Yerushalayim, asked him why he travels on these long journeys, and what is lacking in his avodas Hashem in Teveria. Reb Motke replied: The Mishnah (end of Sotah) states, "When Reb Meir was niftar, it became batel, annulled, the people who knew how to tell parables... when Raban Gamliel the elder was niftar, the honor of Torah was batel, annulled, and purity and prishus died." Notice that by the final words, the Mishnah doesn’t say that prishus became batel, rather that it died. This means טהרה ופרישות is still existent, but in a dead state. Reb Motke explained, "I travel to Slonim because there I learn how to have taharah and prishus with simchah. Because prishus should be alive and practiced with joy. It shouldn’t be dead."