Siman 344

Our Sages, of blessed memory, said (69b) regarding one who travels in the desert and forgot the days and does not know when it is Shabbat, what shall he do: He counts seven days from the day he took his forgetfulness to heart, and sanctifies the seventh with kiddush and havdalah. And for him, the day he forgot begins as the first day, and he counts six and observes one day. And thus our Sages, of blessed memory, said that he has no recognition except through kiddush and havdalah, for regarding melakhah all days are equal to him, as on every day he is forbidden from melakhah lest today is Shabbat. And in this matter, nullification by a majority is not applicable, for behold, every day is fixed in its own right, and furthermore, there is necessarily a Shabbat and he does not know when it is, and even a rabbinic prohibition is forbidden for him to do. And "a rabbinic doubt is ruled leniently" is not applicable in this, for any doubt that arises from a lack of knowledge is not a doubt, as written in Yoreh Deah siman 110. And furthermore, there is no doubt here, for perforce there is a Shabbat here standing on its own and it is not mixed with the other days. However, if he has no bread to eat, perforce he is permitted on every day to do enough for his livelihood for bread, for it is pikuach nefesh and overrides Shabbat. And even on the day that he considers Shabbat for himself he shall do so, for one cannot say that he should make two portions of livelihood from the day before, for perhaps yesterday was Shabbat. And regarding his walking, he may walk all the days as much as he desires, for if not so, he will never reach a settlement. And even though on the day he rests he could refrain from walking more than the techum shabbat, nevertheless, since techumin are rabbinic, they were not stringent upon one lost in the desert (Ran and Maggid Mishneh in chapter 2). And even if techumin are from the Torah according to the one who holds that twelve mil is from the Torah, nevertheless, since there is no prohibition of stoning in this, they were not stringent upon one lost in the desert (ibid.), in order to hasten to leave the desert (Tosafot and the Rosh) and fulfill Shabbat properly. And this is what we said, that Shabbat is not recognized for him except through kiddush and havdalah.

And if he knew the count of the day on which he departed, for example, if he knows that today is the fourth day or the fifth day since his departure, but he does not know on which day he departed—he is permitted to perform work every eighth day from his departure, for he certainly did not depart on Shabbat. And likewise, he is permitted to perform work as much as he desires on the fifteenth day from his departure and on the twenty-second day, and so on forever. And likewise, if he knows clearly that he also did not depart on Erev Shabbat—he is permitted to perform work for two days, the seventh day and the eighth day from his departure, and so on forever. And if it is within his ability to prepare his food then for the entire week—he is obligated to do so, and it is forbidden for him to perform work on the remaining days entirely.