Why is Rosh Chodesh sometimes one day and sometimes two days?

Today is the 30th day of Av (Rosh Chodesh Elul), 5770

The Hebrew months follow the cycle of the moon. The cycle of the moon is approximately 29.5 days, so, there are Hebrew months of 29 days (chaser) and Hebrew months of 30 days (maleh)

Originally, all months were established according to the moon. The English word ‘month’ and the word ‘moon’ are cognates (they share the same etymological origin).

When the ending month is 29 days long, Rosh Chodesh of the new month will always be celebrated for one day: the first day of the new month.

When the preceding month has 30 days, then Rosh Chodesh of the new month will be celebrated for two days. In this case, the 30^th day of the preceding month will be called Rosh Chodesh of the coming month.In the Hebrew calendar normally but not necessarily, a 29 days month will be followed by a 30 days month, etc.

(That use to be also the case with the Gregorian calendar until Augustus Cesar -year 8 BCE- got jealous of Julius Cesar and demanded that the month established in his honor be 31 days long, the extra day was discounted from February…).

Illustration: we are finishing today the month of Av. Av has 30 days. Today is the 30^th day of Av which is ‘also’ Rosh Chodesh Elul. Tomorrow, the 1^st day of Elul will be the 2^nd day of Rosh Chodesh Elul.