It states in sefer Masuk m'Dvash (from Reb Yitzchak Parchi zt'l 6), "We saw, with our own eyes, that those who had a problem when Shabbos arrived, but took their minds off their tzarah and didn't let their worries enter their heart and rejoiced with Shabbos when Shabbos ended, the problem was gone. Everything turned to joy. They were saved from their tzaros with wonderful miracles."
Reb Yaakov Yosef Herman zt'l was from the tzaddikim who lived in America about one hundred years ago. He was an example to all that it was possible to 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 took ill, and Reb Yaakov Yosef and his Rebbetzin stayed with her in the hospital around the clock. As Shabbos approached, they decided to go home to perform their weekly mitzvah of hachnasas orchim.
There was another Herman in the hospital – unrelated 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 the hospital sent a messenger to the Herman home to tell them the unfortunate news. But accidentally, they confused the two Herman families, and the messenger was sent to the house 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. They didn't want to hear or see the letter. It was Shabbos; they would be happy, and that was final.
The messenger left the letter at the home of one of the Herman relatives. The 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. The letter wasn't intended for you. It was written to the other Herman family. Your daughter is doing fine. In fact, over the past twenty-four hours, her health has drastically improved."
Their joy on Shabbos healed their daughter.
I heard from a person who works in kiruv rechokim that he once gave a speech on the topic of being happy on Shabbos, and he repeated stories of tzaddikim who refused to cry or to mourn on Shabbos, even after a great loss, r'l. One person from the audience (not yet religious) became very upset when he heard that. He said, "How can the Torah obligate us to control our emotions? And how can the Torah not permit a person to feel his emotions at a difficult time? Is there no compassion in religion?"
The speaker sent up a silent prayer to Hashem before answering. He told Hashem, "This is for Your honor, not mine. Please send me the right words."
Then he answered, "There was a famous actor who was informed minutes before a show that his father had just died. But the show had to go on, so the actor pushed the sorrow out of his mind and played his part. After the show, the producers announced that the actor had acted superbly, even after discovering that his father had died moments before the show. He had succeeded in overcoming his emotions."
The actor received a standing ovation from the crowd.
In this manner, he gave the questioner to understand that there are times when one must overcome his emotions. Shabbos is such a time. Shabbos is a time of joy.
Here is another example of someone who experienced yeshuos by being happy on Shabbos.
One 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 on top of each 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 things will improve. However, if they don't, we will need to operate."
It was hard for this yungerman to be happy that Shabbos due to his pain and his fear of surgery. His melancholy feelings affected the mood in his home. No one wanted to sing zemiros, and the delicious Shabbos food tasted bland.
But suddenly, the yungerman jumped up from his seat and grabbed a Gemara Bava Metzia. He opened it to Daf 25 and, with a shining face, repeated the Gemara numerous times. His family watched on in amazement, not understanding how his mood changed so quickly.
After studying the Gemara a few times, he told his family, "The Gemara discusses a case where someone found three coins of different sizes, one on top of the other. The largest coin was at the bottom, a medium-sized one in the middle, and a tiny one 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 suddenly remembered this Gemara and realized that my kidney stones are in the same formation: the largest stone at the bottom, the medium in the middle, and a small stone on top. As the Gemara explains, this formation means the stones didn't reach there by chance. Obviously, they were placed there purposely by Hashem. And since it is from Hashem, I know it is for my good. Now I can be happy."
A week later, the stones left his body naturally. The joy of Shabbos brought him a remedy.