Rif Shabbat 31a

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31a

BEGIN PEREK SEVEN
{Shabbat 67b continues}
MISHNA:
A GREAT PRINCIPLE WAS STATED IN RESPECT TO THE SABBATH:
HE WHO FORGETS THE FUNDAMENTAL LAW OF THE SABBATH AND PERFORMS MANY LABOURS ON MANY SABBATHS, INCURS ONE SIN-OFFERING ONLY.
HE WHO KNOWS THE FUNDAMENTAL LAW OF THE SABBATH AND PERFORMS MANY LABOURS ON MANY SABBATHS, INCURS A SIN-OFFERING ON ACCOUNT OF EACH SABBATH.
HE WHO KNOWS THAT IT IS THE SABBATH AND PERFORMS MANY LABOURS ON MANY SABBATHS, IS LIABLE FOR EVERY PRIMARY LABOUR. HE WHO PERFORMS MANY LABOURS BELONGING TO THE SAME CATEGORY OF WORK IS LIABLE TO ONE SIN-OFFERING ONLY.

{Shabbat 69b}
Gemara:
Rav Huna said: If one is travelling in the wilderness and does not know when it is the Shabbat, he must count six days and observe one.
{our gemara had "on the road" besides "in the wilderness" - but Rif, Rosh, Rambam, Tur, etc., all omit}

Rava said: Every day he does sufficient for his requirements [only], except on that day.
And on that day he is to die?!
He prepared double his requirements on the previous day.
But perhaps the previous day was the {actual} Shabbat?
Rather, every day he does sufficient for his requirements, and even on that day.
Then wherein may that day be recognized?
By kiddush and havdalah.

Rava said: If he recognizes the relationship to the day of his departure {even though he does not know what exact weekday he set out}, he may do work the whole of that day.
But that is obvious?
You might say -- Since he did not set out on Shabbat, he did not set out on the eve of the Sabbath {that is, Friday} either; hence this man, even if he set out on Thursday. it shall be permitted him to do work on two days.
Hence he informs us that sometimes one may come across a company and chance to set out [on a Friday].