Determined by the new moon

Rosh Hashanah observances vary. Orthodox Jews celebrate Rosh Hashanah for two days. Reform Jews often observe it for only one day. In Biblical times the moon, not the calendar, determined dates for festivals. Witnesses watching the sky proclaimed the new moon. Since Rosh Hashanah falls on the first day of the month, people living far from Jerusalem did not have time to learn the exact date. Even those living near Jerusalem could miss the festival if the witnesses did not arrive on time. So, two days were set aside for observance so everyone would have time to participate.

The new moon is expected to be spotted in Jerusalem on Wednesday Sept 12th.   Which is actually Thursday in Israel...  Jewish Weddings occur on Wednesday in Israel, and wouldnt it be ironic if this was the day of the rapture.....
Below is from a website   http://www.factmonster.com/spot/roshhashanah1.html

On Thursday night, Sept. 12, 2007, Jews will begin celebrating one of their most important religious holidays, Rosh Hashanah. It remembers the creation of the world. In Hebrew, Rosh Hashanah means the "head of the year." It is also called the Feast of the Trumpets. The blowing of a ram's horn, a shofar, proclaims Rosh Hashanah, and summons Jews to religious services. Sundown Sept. 12 marks the beginning of the year 5768 and the month of Tishri.

Jews used the ram's horn as a trumpet in Biblical times to announce the new moon, holidays, and war. Today, a variety of horns are used, including curved antelope horns.

Putting your spiritual house in order

While it does have its festive side, Rosh Hashanah is not one big party, as the New Year's celebrations on Dec. 31 tend to be. Rosh Hashanah is a time for personal introspection and prayer.

Jews may also visit graves. It is thought that the prayers or good wishes of the dead can help the living. By wishing each other well and sending cards, people let friends know what happened in the past year and what plans lie ahead. Christmas cards and get-togethers fill a similar role for Christians.

Rosh Hashanah is part of a process of spiritual growth. The Hebrew month preceding it, Elul, is a time for charity, tzedakah. Rosh Hashanah falls on the first and second days of the seventh month, Tishri.

Rosh Hashanah Dates (beginning at sundown on the following dates)
2005	October 3
2006	September 22
2007	September 12
2008	September 29
2009	September 19