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On Shabbat, all melakha is prohibited whereas on Yom Tov, some melakha is prohibited and some is permitted. We will now explain.
On Yom Tov, one may not reap grain, harvest grapes, grind sheaves, or press grapes into wine or olives into oil. All of these melakhot are deemed melekhet avoda (activity connected with working) and are forbidden on Yom Tov. The indication that a particular activity is melekhet avoda is that it is generally done by hired workers, who prepare large quantities of produce to last a long time or for commercial purposes. However, on Yom Tov one may knead dough, bake bread and cake, and cook food. Additionally, food for a meal may be carried from one domain to another. Unlike melekhet avoda, food preparation is generally done at home, for that day’s needs. The relevant melakhot are permitted to enable people to enjoy the festival. They may not be done for later weekday use or for non-Jews.
Fire may be transferred on Yom Tov, as such it is permitted to light a match from a lit candle and use that flame to light a burner. One may also turn up the burner flame on a stove. However, the Sages prohibited lighting a new fire on Yom Tov, because creating something new is too similar to doing a prohibited melakha. This prohibition includes turning on an electric light or oven.
One may not extinguish or lower a flame on Yom Tov. However, lowering a flame is permitted if it is done in the course of food preparation. Therefore, raw meat may be placed over coals, even though the juices dripping from the meat will put out part of the flame. Similarly, the flame under a pot of food may be turned down if it is likely to burn the food.
In the past, people who wanted to cook on Yom Tov would put enough firewood in the oven to allow them to cook everything necessary. This wood would be lit on Yom Tov with a flame prepared before the festival. When the cooking was done, the fire would burn itself out. Today’s equivalent is barbecuing.
Gas burners are different. The flame will not go out by itself, and turning off the gas is forbidden. However, one is permitted to put a flame out if it can be done indirectly. For example, the Zomet Institute in Israel markets a Holiday Gas Timer (“Chagaz”), a device invented specifically for cooking on Yom Tov. It involves a spring-operated mechanical timer. While it is on, gas flows through the pipes and feeds the flame; at the pre-set time, the gas supply is cut off, causing the flame to go out. Before people start to cook, they set the timer for the desired amount of time. When the time is up, the Chagaz simply cuts off the gas.
