English Translation
This is a special occasion.
Gemini says:
Yes, “Il s’agit d’une occasion spéciale” is grammatically correct but it sounds very formal and administrative.
It translates closer to: “It is a question of a special occasion” or "We are dealing with a special occasion."
If you are at a party, raising a glass, or simply stating that today is important, you should use C’est.
C’est une occasion spéciale. (This is a special occasion.)