1) Auspicious 2) Critical 3) Crucial 4) Historic 5) Important 6) Of very great significance 7) Significant 1) Big 2) Earthshattering 3) Epochal 4) Eventful 5) Fateful 6) Historic 7) Important 8) Serious 9) Significant 
• (a.) Of moment or consequence; very important; weighty; as, a momentous decision; momentous affairs.
Mo·men'tous adjective [ Confer Latin momentosus rapid, momentary.] Of moment or consequence; very important; weighty; as, a momentous decision; momentous affairs. -- 
adjective of very great significance; `deciding to drop the atom bomb was a very big decision`; `a momentous event`
of great or far-reaching importance or consequence: a momentous day.
[SAT terms] of very great significance of very great significance