The Jewish calendar today is set based on calculations made by Hillel II in the time of the Gemara. Each month has either 29 or 30 days, so that over time, the months will stay in sync with the moon. As we have learned, during Talmudic times, the new month was based on testimony received from witnesses who saw the new moon, although the Sages who declared the new month had a fair amount of latitude to choose to postpone the announcement if they felt it necessary for one reason or another.
Rabbenu Ḥananel and the Ge’onim point out that a decision on establishing the new month was dependent on a number of issues, some of which were well known, but others were known only to a small group of Sages who participated in sod ha-ibur – the closed assembly that actually made the final decision on this matter.
It is related that when Ulla came from to Bavel, he said: This year they added an extra day to the month of Elul. Ulla continued and said: Do our Babylonian colleagues understand what benefit we did for them? We pushed off Rosh HaShana for a day, so that the Festival would not occur adjacent to.
By keeping apart two holy days on which a great many creative activities are forbidden, we avoid such problems as how to deal with a dead body that cannot be buried for two days (as explained by Rabbi Aha bar Hanina) or the problem of how to ensure that there are fresh vegetables available to eat on Shabbat if it immediately follows Yom Kippur (Ulla’s explanation).
While these explanations seem very logical, the Gemara asks why Ulla considered this to be a boon for the Babylonian Jewish community more than for the Jewish community residing in Israel. The Gemara responds with a simple explanation – the temperatures are higher in Bavel than in Israel, so the concerns of spoilage over two days are much greater there.
The weather in Bavel at the end of the summer – particularly between the Tigris and Euphrates Rivers, which is where the Jewish community lived – is 3°-5° Celsius (5°-10° Fahrenheit) higher than in Israel at that time of year. Moreover, there is little rainfall, and the distance from the ocean is so great that there are no sea breezes there. Thus, the weather in Bavel was considered to be much hotter than in Israel.