This banner is directed to every visitors of our website, not only to the conference attendees. Therefore, "welcome to the XXX conference" sounds a little inappropriate to me. I share the feeling that "grand" sounds weird, but I could not think of another word. How about "Coming soon, XXX conference"? The word "congratulations" is added to create an auspicious and lively atmosphere. I think it's used to congratulate the successful opening of the conference.