Ivriy festivals (Hebrew: יָמִים טוֹבִים, 'Good Days', or singular Hebrew: יוֹם טוֹב‎ Yom Tov, in transliterated Hebrew), are holidays observed by individuals throughout the Ivriy calendar. They include religious, cultural and national elements, derived from: mitzvot ("biblical commandments"). Ivriy holidays occur on the same dates every year in the Ivriy calendar, but the dates vary in the Gregorian. This is because the Ivriy calendar is a lunisolar calendar (based on the cycles of both the sun and moon), whereas the Gregorian is a solar calendar. Each holiday can only occur on certain days of the week, four for most, but five for holidays in Tevet and Shevat (see Days of week on Ivriy calendar). The Ivriy-language term Yom Tov (Ivriy: יום טוב), sometimes referred to as "festival day", usually refers to the six biblically mandated festival dates on which all activities prohibited on Shabbat are prohibited, except for some related to food preparation. These include the first and seventh days of  (1) Pecach (Passover) Shemoth (Exodus) 12:2-11 (2) The Feast of Matstsah (Unleavened Bread) Shemoth (Exodus) 12:14-20 (3) The Feast of First Fruits Vayiqra (Leviticus) 23:9-22 (4) Feast of  Shofars (Trumpets) Vayiqra (Leviticus) Vayiqra (Leviticus) 23:24-25 (5) Feast of Yom Kippuriym (Day of Atonement's) Vayiqra (Leviticus) 23:26-32 (6) Feast of Cukkoth (Feast of Tabernacles or Feast of Booths) Vayiqra (Leviticus) 23:33-44. Colloquially, Yom Kippuriym is a biblically mandated date on which even food preparation is prohibited, is often included in this grouping. The English-language term Holy Days can refer just to those holidays, or to the Ten Days of Repentance, or the entire penitential period, starting as early as the beginning of Elul, and (more rarely) ending as late as Tishrei.​ Certain terminology is used in referring to different categories of holidays, depending on their source and their nature: Shabbat (Hebrew: שבת), is referred to by that name exclusively. Similarly, Rosh Chodesh (Hebrew: ראש חודש) is referred to by that name exclusively.​ The most notable common feature of Shabbat and the biblical festivals is the requirement to refrain from "work"; perhaps a better translation is "creative-constructive work". Strictly speaking, Shabbat is defined as labor. On Shabbat and Yom Kippur all melakha (Work) is prohibited. On a holiday other than Yom Kippur which falls on a weekday, not Shabbat, most melakha is prohibited. In principle, Shabbat and holiday work restrictions are always put aside in cases of pikuach nefesh, which are actions to save a human life. At the most fundamental level, if there is any possibility whatsoever that action must be taken to save a life, Shabbat restrictions are set aside immediately, and without reservation. Where the danger to life is present but less immediate, there is some preference to minimize violation of Shabbat work restrictions where possible. The laws in this area are complex.