Anywhere and everywhere Jews live.
Some certainly only celebrate Rosh Hashanah because they feel guilty otherwise, but most Jews choose to celebrate Rosh Hashanah because they want to engage with their history and traditions.
partying
Reform Jews outside Israel generally celebrate Rosh Hashanah for only one day, while all other Jews celebrate Rosh Hashanah for two days. Reform Jews blow the Shofar on Rosh Hashanah even if it falls on Shabbat (the Sabbath), while others refrain from blowing the shofar on Shabbat.
Jewish New Year
Rosh Hashanah was a Biblical holiday, so it was celebrated for centuries before the birth of Jesus Christ and has been celebrated continuously since then.
Rosh Hashana is the only Jewish holiday that is two days long both in and out of Israel. See the question "why does rosh hashanah last for two days"
they meet as a group to pray
Rosh Hashanah.
Rosh Hashanah celebrates the New Year, the anniversary of the date that God created Adam and Eve.
Rosh Hashanah 2008 begins on sunset of Monday evening, September 29. Orthodox Jews celebrate 2 days of Rosh Hashanah, Reform Jews celebrate 1.
Rosh Hashanah is the Jewish New Year.
Rosh Hashanah was on October 3rd in 1959.