In this week's Parsha, the Torah states "לא תבערו אש בכל משבתיכם ביום השבת," "do not kindle a fire in any of your dwelling places on the Shabbos day" (שמות ל"ה:ג'). The Shaloh writes that besides its literal meaning, the Posuk is also referring to the "fire" of anger and disputes. One should thus not be angry on Shabbos. Ideally, one should avoid becoming angry at all times. The reason that Shabbos is specifically mentioned is that when one is together with one's family at the Shabbos table, parents may become angry with their children for misbehaving; with more personal interaction on Shabbos, the opportunity for disputes becomes greater, and the Torah therefore teaches us to be especially vigilant.
The Torah's statement applies to Friday as well. When one is in a rush to finish preparing for Shabbos, one is likely to become short-tempered. In conjunction with this notion, the Gemara in Shabbos (דף ל"א.) tells a story about Hillel HaZakein. A wager was once made between two people stipulating that whichever one would make Hillel angry would receive a large sum of money from the other. It was Erev Shabbos, and one person claimed he could make Hillel angry. While Hillel was washing himself for Shabbos, the man stood at the front door of where Hillel was and called out "Where is Hillel, where is Hillel?" Wrapping his robe around himself, Hillel ran to the door to greet the man. Hillel asked him what it was that he required. "I have a question," the man replied. "Please ask whatever you want," said Hillel. "I wish to ask why Babylonians have round heads." "You have asked an important question," Hillel replied patiently. "The reason is that their midwives were not clever." Satisfied, the man left the house, waited until Hillel resumed washing himself, and again called out "Where is Hillel, where is Hillel?" Hillel again ran to the door and patiently answered the man's next ridiculous question. The man left and repeated the procedure a third time. Once again, Hillel calmly answered the man's absurd question. The man then said "I have a lot of questions, but I fear that you will become angry." "Please, feel free to ask as many questions as you wish," Hillel replied. "May there not be many men like you," the man said. "Because of you, I have lost a lot of money." Hillel then told the man, "it is better for you to have lost a lot of money than for me to have lost my temper."
We thus see that not only does the Torah prohibit kindling a fire on Shabbos in the literal sense, but the Torah also teaches us to control our tempers at all costs. This is especially true on Friday and Shabbos when avoiding arguments becomes an even greater challenge. We must avoid having anger burn inside us, especially on Friday and Shabbos when the prohibition is even greater.