In honor of the Gregorian new year, here is a post on technical calendar issues. It's not often that the details of the lunar calendar play a central role in a major international news story!
Back when the Jews still lived primarily in the land of Israel, the lunar calendar was based on observation of the new moon. When a set of witnesses saw the new crescent moon, they would testify before the Sanhedrin that they had seen it, and the Sanhedrin would declare the new month. All of the holidays would be set based on this declaration.
Mishnah Rosh Hashanah 2:8 deals with various cases when the content of the witnesses' testimony is questionable. In one case in the 2nd century CE, a pair of witnesses came to Yavneh and said that they had seen the moon on the 30th night (the night that could be the 1st of the new month if the new month was declared), but then didn't see it on the following night. Rabban Gamliel (the presiding officer of the Sanhedrin) accepted their testimony. Rabbi Dosa ben Harkinas had a dissenting opinion: These witnesses are liars! How can they testify that a woman has given birth, and then the next day she is still pregnant ("her belly is between her teeth")? Rabbi Yehoshua joined in this dissent.
In the following mishnah (2:9), things start to get heated between Rabban Gamliel and R. Yehoshua. You see, if they disagree about whether to accept these witnesses' testimony, then they'll disagree on the date of Rosh Chodesh (R. Yehoshua would put it one day later) and thus on the dates of that month's holidays. That month was apparently Tishrei, with lots of important holidays. R. Gamliel sent R. Yehoshua a message: "I order you to come to me with your staff and your money, on the day that would be Yom Kippur according to your accounting." R. Gamliel and R. Yehoshua would have agreed that carrying these things into the public domain was forbidden on Yom Kippur (but just fine on 11 Tishrei, a regular weekday). So R. Gamliel was in essence commanding R. Yehoshua to accept R. Gamliel's opinion about when the holidays fall, and to announce this in public by doing something he would never do on Yom Kippur. After some angst and soul-searching about this, R. Yehoshua capitulated and went to R. Gamliel.
Fast forward eight centuries. Most of the Jewish population now lived outside Israel, and there was no Sanhedrin. The system of declaring the new moon based on the testimony of witnesses was gone, and had been replaced by a mathematical algorithm. One of the features of this algorithm is the four dechiyot -- circumstances that require Rosh Hashanah to be moved 1 or 2 days later than the calculated date of the new moon. One of the dechiyot, called molad zakein, says that if the molad (lunar conjunction) happens later than 18 hours (= noon, since we start counting from sundown), then Rosh Hashanah is delayed to the next day. [There are several possible reasons for this, which could be addressed in another post upon request.] Aharon ben Meir, of Eretz Yisrael, proposed changing this rule to 18 hours 642 minutes (=12:36 PM instead of noon), and changing two of the other dechiyot by the same amount. [Again, there are several possible reasons for this change.] Saadiah Gaon, of Babylonia, opposed the change. The molad of Tishrei in the year 923 CE happened on Shabbat at 18 hours 237 parts (12:13 PM). Therefore, according to Saadiah Gaon, since this was after 18 hours, Rosh Hashanah was delayed by a day, and then delayed by another day since Rosh Hashanah can't fall on Sunday, so he (and those who followed him) observed Rosh Hashanah on Monday. Aharon ben Meir (and those who followed him) observed Rosh Hashanah on Shabbat. Similar issues applied to Rosh Hashanah of the previous year. Therefore, the parts of the Jewish world that followed Aharon ben Meir observed all the holidays in 922 and 923 CE two days earlier than the parts of the Jewish world that followed Saadiah Gaon.
Fast forward to the present time. The molad zakein stayed at 18 hours, and Jews around the world agree on how to calculate the calendar. The Muslim world, however, is a different story. Islam also uses a lunar calendar, and in many countries, Muslims still rely on witnesses who observe the new moon, rather than a calculation. (In North America, the system just switched over this year!) Of course, this system has gone high-tech, and the Internet and telephones are much more efficient and reliable ways of transmitting the information than signal fires or messengers.
There is no internationally recognized Muslim authority (just as there has been no internationally recognized Jewish authority since the Sanhedrin ended). This means that Muslim communities in different countries, and sometimes within the same country, will disagree on the dates of the holidays, because they disagree about using calculation vs. observation, or because they're using observation but simply have different witnesses providing different data.
This year, Iraqi Sunnis and Iraqi Shiites disagreed on the day that the month of Dhu al-Hijjah began. (Does anyone know the details? Are they using different systems, or are there just different authorities accepting witness testimony?) As a result, the festival of Eid ul-Adha (which begins on the 10th of the month, just like Yom Kippur or 10 Nisan, or appropriately for this year, 10 Tevet), began on Saturday, December 30, for Iraqi Sunnis, and Sunday, December 31, for Iraqi Shiites (and in North America).
Saddam Hussein was executed on Saturday morning. All relevant parties agree that executions should not take place during Eid. But the Shiites said that Saturday was not Eid, so the execution was permissible that day. Saddam Hussein was secular except when he had to appear religious for PR purposes, but he was of Sunni heritage.
All three of these stories have something in common: while the controversy (Rabban Gamliel vs. Rabbi Yehoshua, Aharon ben Meir vs. Saadiah Gaon, Sunnis vs. Shiites) is formally about technical calendar issues, deep down it's really about power politics.
In the Mishnah, the root of the controversy was the authority of the Sanhedrin, particularly R. Gamliel's personal authority, vs. the freedom to hold dissenting opinions. R. Gamliel was not willing to brook dissent, a trait that would eventually get him impeached and removed from office (and R. Yehoshua would recall this calendar controversy on that famous day). In compelling R. Yehoshua to carry in the public domain on a particular day, R. Gamliel wasn't just asserting a particular stance about the procedure for declaring the new moon, but was asserting his own centralized rabbinic authority and was compelling R. Yehoshua to accept that authority. Winner: R. Gamliel in the short term, but R. Yehoshua gets the last laugh when R. Gamliel is overthrown. Today there is no Sanhedrin, so centralized authority loses in the long term.
In the 10th century CE, the controversy was about Israel vs. the Diaspora. Saadiah Gaon was asserting his own authority as the premier Torah scholar of his time (from the Diaspora, where Jewish life was centered), and Aharon ben Meir was asserting the primacy of Eretz Yisrael. Winner: None in the short term (since both have their followers), but Saadiah Gaon in the long term -- we still follow his opinion about the calendar (which will next become an issue in 2025 CE, not so far in the future), and until the last century, Judaism was Diaspora-based.
In our time, the controversy is about who controls Iraq in the chaotic post-Saddam era. By executing Saddam on the Sunni date of Eid, Iraq's Shiite majority was asserting power and authority that had been denied to it under the (secular Sunni) Baath regime. This was as clear a signal as R. Gamliel's order to R. Yehoshua, asserting R. Gamliel's authority. But Iraq's Sunni population is not going to respond as deferentially as R. Yehoshua did. Winner: The Shiites in the short term. In the long term, God help us all.