A. Biblically, its a one-day holiday, the first of Tishrei. Practically, its observed as a tw0-day holiday in nearly all Jewish communities.
Most two-day Jewish holidays (such as the first two and last two days of Passover and Sukkot) are kept as one day in Israel. This dates back centuries to the times of the Mishna. However Rosh Hashanah is an exception, and even Israel keeps it as a two-day holiday.
Most Reform congregations observe only one day of Rosh Hashanah.