Beitzah 16 - Fixed income
Person's available income for the year is fixed and is decided on Rosh Hashanah, and therefore one should not make extravagant purchases - because his income for food won't be increased. However, he can freely spend more for the needs of Shabbat and Yom Tov, and also for the Torah education of his children - because the extra amounts will be added to his allotment. Shammai would always buy things in honor of Shabbat, from the first day of the week, and if he saw something better - he would buy that too, and eat the previous one on a weekday. Hillel, on the other hand, was unconcerned, trusting that God will provide him with the needs for Shabbat when the day comes.
If one gives a present to his fellow, he need not inform him, because it will become known to his fellow anyway, and the goal of increased friendship will be achieved. Why then God, when giving people the Shabbat, told Moses to inform them about it - even though they were soon to experienced it!? - True, but God told Moses to inform them of the hidden qualities of Shabbat: the reward that one gets for it, and the extra soul that one gets on Shabbat and that allows him to eat more with pleasure.
Back to the subject of "eruv" or food mix which permits one to cook on a Holiday for the needs of Shabbat - one can use any food for that, even a remainder of a dish. Even if he eats it on a Holiday, it still has the legal effect, provided that there is a little of it left.
Art: The feast by Adriaen Brouwer