Just about every synagogue in the world has both an evening and a morning service on Rosh Hashanah (although in this age of COVID-19, many liberal synagogues will use Zoom.) The liturgy for Rosh Hashanah has special melodies that are only used on this day, and the morning service is greatly expanded with penetential prayers and, most notably, a section where the shofar, a ram's horn Trumpet, is blown repeatedly in a scripted series of stuttering, broken and long blasts. If you count the total number of toots it is at least 100. Rosh Hashanah is a feast day, so a festive evening meal is the norm, as well as a festive kiddush lunch after the morning service. The challah (bread) served at the meals is traditionally round instead of being a long braid, and the meals traditionally include sweets to symbolize a sweet new year. Apples and honey, honey cake and similar foods are symbolic. On the afternoon of Rosh Hashanah, it is traditional to go down to the riverbank or seaside and empty your pockets -- symbolically emptying them of sin, but in reality, throwing bread crumbs on the water.
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.
Anywhere and everywhere Jews live.
Jewish New Year
they meet as a group to pray
Rosh Hashanah.
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"
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.