Sfeka d'yoma
Sfeka d'yoma (Talmudic Aramaic: ספיקא דיומא, lit. "doubt about the day", or doubt regarding the exact date of the day) is a concept and legal principle in Jewish law which explains why some Jewish holidays are celebrated for one day in the Land of Israel but for two days outside the Land. The implications of "sfeka d'yoma" are discussed in Rosh Hashanah 21a and in the commentaries and poskim. Background. Before the destruction of the Temple in Jerusalem, and prior to the permanent establishment or "fixing" of the Hebrew solar-lunar calendar, the onset of each month, known  as Rosh Chodesh (, lit. "head/start of the month/new moon") was declared by the Sanhedrin. This declaration was crucially important, as it affected the dates on which the holidays in that month would occur. The Sanhedrin would declare a new month only after accepting the testimony of two witnesses who attested to the sighting of the new moon. If the witnesses were contradicted, or not enough witnesses came forth, the Sanhedrin would convene on the following day to receive testimony. Upon accepting the testimony of two witnesses, the Sanhedrin would sanctify the new month. Messengers would then be sent  throughout the Land of Israel, and bonfires lit on high places, to let everyone know the correct date of the new moon. While this system insured that Jews living in Israel would immediately know the date of the new month and, by extension, the dates of the holidays that fell in that month, the news would travel slower to Jews living outside the Land – particularly in Babylonia, where there was a large Jewish population following the destruction of the First Temple and during the era of the Second Temple. Hence, the dates on which the Jewish holidays would occur in  each month would be uncertain until such time as every community would know exactly when the new month had been declared by the Sanhedrin. Since there was doubt about which day to observe the holidays, it was established that the holiday, or Yom Tov, be celebrated on two days rather than one. Based on the nature of the start of the lunar months, there could evidently be a margin of error of only one day. Fixing of the calendar. A few centuries after the destruction of the Second Temple and the worldwide exile of the Jewish people, the Jewish community  in Israel began to decline. The calendar was then fixed (meaning that it was permanently set up by the Jewish sages). With this fixed calendar, there is no doubt about which day the holidays in each month occur. Nevertheless, Jewish communities outside the Land of Israel continue to celebrate two days for certain Jewish holidays because that is the "minhag" (custom) that the Jewish sages established during an era when it was needed. This is referred to as "minhag avoteinu b'yadenu" (, lit. "the custom of our forefathers is in our hands") – meaning that Jews have traditionally accepted and practiced what  their ancestors have handed down to them, according to the long-standing classical view that established customs may not be changed. Holidays that reflect "sfeka d'yoma". "Sfeka d'yoma" is incorporated into all holidays mentioned in the Torah (Pentateuch) with the exception of Yom Kippur. "Sfeka d'yoma" was not instituted for holidays that are not mentioned in the Torah but were established later under rabbinic law. These include: