In the past on Celebrity when they way "informal night" it turns into "casual Night" and my husband has not needed a tie and jacket. If we go on the Millennium in April is he fine with just packing a suit for formal night?
Boy, they sure use confusing terms. I don't know which nights on your specific cruise are informal or casual, so I suggest you call the cruise line just to make sure. It's always a good idea to take along an extra jacket I think, especially if you plan to dine in the elegant alternative restaurant.
You might want to post this over on the Celebrity board as this is the Crystal board and the two lines are quite different in terms of dress. But, in short, I just returned from a Celebrity cruise and for informal/semi formal nights, the men wore jackets, nice shirts and no tie. Casual nights are just slacks and a collared polo-type shirt.