The Sanhedrin would declare a new month only after accepting the testimony of two witnesses who attested to the sighting of the new moon.
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.
This is referred to as minhag avoteinu b'yadenu (Hebrew: מנהג אבותינו בידינו, lit.
Sfeka d'yoma is incorporated into all holidays mentioned in the Torah (Pentateuch) with the exception of Yom Kippur.