1. Why the orthodox have an extra day of Festivals – and we don’t – usually!
Rabbi Jonathan Keren-Black, Leo Baeck Centre
Introduction to Judaism course 5777

2. New Moon – New Month

3. A Special Rosh Chodesh
Rosh Chodesh – Chodesh is from ‘Ch D Sh, ‘new’, and refers to month, as each new moon brings a new month. Rosh means ‘Head’, so Rosh Chodesh is Head of the Month, or ‘New Month’.
Every month of the year is announced in synagogue on the Shabbat before it arrives, or when it arrives. But there is one exception – the new month of Tishrei. It is not announced in the same way.

4. One day or two? Lunar months are about 29.5 days long
Therefore months may have 29 or 30 days. After 29 days, if a new moon appears, then it is the first day of the new month. But if it doesn’t, then that day is counted as the thirtieth day of the previous month.
Within ancient Israel, a Bet Din would determine the day (by examining witnesses, or if there weren’t any, by other means), and would send the message across the land by bonfires or by messengers on horseback.
Outside of the land it was less certain whether the new moon had been announced, and whether the message had got through reliably. To be on the safe side, the two possible days were both observed – hence 2 days!

5. An example – the months of Iyar/Sivan
First possible day in the month – –10 etc
Other possible day of month –29-30 – 1 – 2 – 3 – 4 – 5 – 6 – 7 – 8 – 9 etc
With Festival (Shavuot on 6th Sivan) X
But in case they are wrong: X
Therefore orthodox have 2 days: X X
Orthodox observance of month – 1 – 2 – 3 – 4 – 5 – 6 – 7 – 8 – 9 etc – 2 days

6. Another example – Nisan and Pesach
First possible day in the month – – – 22 – 23 – 24 etc
Other possible day of month – 13 – 14 – 15 – 16 – 17 – 18 – 19 – – 22 – 23 – 24
With Festival (Pesach on evening of 14th Nisan for 7 days)
(7 days =1 week)
First possible day in the month – – – 22 – 23 – 24 etc
(but in case they are wrong: )
Other possible day of month – 13 – 14 – 15 – 16 – 17 –18 – 19 – – 22 – 23 – 24
Therefore: (8 days in total)
Orthodox observance of month 12 – 13 – 14 – 15 – 16 – 17 –18 – 19 – – 22 – 23 – 24

7. Zikhron Teruah – one day in the Torah
In Torah, Leviticus 23: 24, the first DAY of the seventh month shall be Zikhron T’ruah – a Memorial of Blowing
Even in ancient Israel, they couldn’t examine and promulgate the first day of Tishrei in time.
So even in Israel today, 2 days are marked for Rosh Hashanah.
In some diaspora communities the Progressive movements do the same.
In others they do not, but follow Torah and mark just one day Rosh Hashanah.