Why is Rosh Hashanah celebrated over two days? When the ancient Israelites started celebrating the new year, it was a one-day holiday. With no wall calendar to indicate the actual day to celebrate, smoke signals were used to alert scattered communities when it was time to start the holiday. So the sages created a two-day holiday to ensure that Jews everywhere were celebrating at about the same time. "The only part of the original recipe that we've retained" is the practice of observing the holiday for 48 hours, says Mark Leuchter, director of Jewish Studies at Temple. "Now we do it not because we have to but because we used to. It ties us back to a hallowed antiquity."