A celebratory or momentous event, situation, or circumstance. (Note that the meaning of auspicious here is slightly altered from its standard definition of favorable or timely.) We would like to invite you to the auspicious occasion of the marriage of our son, Luke, to his bride-to-be, Sophia. It was an auspicious occasion in the city, as people poured onto the streets to celebrate the queen's ascent to the throne.
See also: occasion