Rabbi Shimon ben Elazar ruled: “A woman may fill her entire oven with bread [when baking on a festival day] since bread bakes better when the oven is full”
Talmud, Beitzah 17a
The festivals are days of rest on which all creative work is forbidden, as on Shabbat. However, there is a major difference between the festivals and Shabbat: on the festivals it is permitted to do all work that pertains to the preparation of food (kindling a fire, cooking, etc.), as long as it is food that is needed for the festival.
This applies also when the labor involved only improves or enhances the food, hence, the above-quoted law: since bread bakes better when the oven is full, it is permitted to bake an entire ovenful of bread, even if only a single loaf is actually needed for the festival.
Often, a careful study of one’s daily schedule can be extremely disheartening. So much time and energy expended on earning a living; so many hours each day devoted to eating, sleeping and other bodily needs; so much squandered on one’s social obligations-how much time is left to attend to one’s true needs and desires? How much time is left for prayer, study, and other spiritual pursuits, for which one’s soul descended into physical life in the first place?
Says Rabbi Shimon: If a dozen baking loaves contribute to the quality of a single loaf, they are all a legitimate part of the single loaf’s baking. If a person makes his spiritual moments the focus of his life, regarding all else as but the means to their end, then his entire day is a singularly holy endeavor.
Based on an address by the Rebbe, Cheshvan 20, 5736 (October 25, 1975). 
Adapted from the teachings of the Rebbe by Yanki Tauber
 Reshimot #19, pp. 7-10.
 With important exceptions; see Laws of the Festivals in Shulchan Aruch, Orach Chaim, sections 495-429.
 Likkutei Sichot, vol. XV, pp. 116-117. See Ethics of the Fathers 2:12, and Mishneh Torah, Laws of Human Character 3:3.