The blessing said whenever anything that has not happened for a while is performed. This includes all holidays except the sabbath, and only the first night of channukah.

Baruch atah Adonai, Elohaynu, melech ha-olam, she-hecheeyanu v'keey'manu v'heegeeyanu la-z'man ha-zeh.

Blessed are you, Lord, our God, king of the universe, who has kept us alive, sustained us, and enabled us to reach this season.