Eruvin 39 - Making conditions
Rosh HaShanah - New Year - is celebrated for two days. Even though the Torah, of course, decreed only one day of Rosh HaShanah, practically it was very hard to know when this day occurs - on the last day of the previous month, or the first day of the next one. So as not to do work on Rosh HaShana by mistake, people abstained from work on the first possible day of Rosh HaShana, and then also on the second.
If one wants to wants to go beyond two thousands steps in different directions on two days of Rosh Hashanah - he can, provided that he places his eruv food in appropriate places, since after all, only one day is the real festival - this is the opinion of Rabbi Yehudah. The Sages agree that it is one festival, but they argue that its long-term observance for two days made both days into one long Rosh HaShanah, so that he cannot have two different eruvs.
Rabbi Yehudah also allows to set other conditions for Rosh HaShanah. For example, if one has two baskets of fruit from which tithes have not been given, he can say on the first day of Rosh HaShanah, "If the festival is tomorrow, then I am separating tithes from one basket with the fruit from the other, but if the festival is today, I have not said anything." Then he makes a similar condition on the second day and eats the fruit. Again, the Sages say it is all one long Rosh HaShanah. However, even the Sages allow such conditions to be made for the two days of a Yom Tov festival, since this second day is observed only outside of Israel and is thus not a universal requirement.
Art: James Peale - Fruit Still Life with Chinese Export Basket
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