The Talmud actually discusses what to do if one is uncertain which day of the week it is: “Rabbi Huna said: If one is traveling on a road or in the wilderness and does not know when the Sabbath is, he must count six days and observe one. Hiyya ben Rab said: He must observe one and count six [weekdays]” (Shabbat 69b).
The question remains, does one begin observing Shabbat immediately or counting immediately? The Talmud explains further the rationale behind each of these opinions.
“Wherein do they differ? One Master holds that it is like at the time of the world's Creation; the other Master holds that it is like [the case of] Adam” (Shabbat 69b).
Since Adam, the first human being, was created on the afternoon of the Sixth Day of Creation, a short while before God rested (the first Shabbat), acting like Adam would mean celebrating the Shabbat at the first sunset. Counting seven days, on the other hand, emulates God, and the idea that is at the heart of the celebration of Shabbat.
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So what should one do if one has no means of knowing what day of the week it is? According to halacha (Jewish law), one should emulate God and begin counting six days, and only then begin to celebrate Shabbat.