Time spent at sea is wonderful -- and even more so if it's aboard a cruise ship!
If I remember correctly, Celebrity's ships take a pretty southern route so summer clothing probably will be appropriate for most of the transit. Of course, it would be prudent to bring some spring/fall clothing, too, in case you encounter some unusually cool weather. On the European side, you might encounter some cooler weather depending upon your choice of itinerary and specific ports of call. I would not expect really cold weather in May unless you are going to Scandanavia or the Baltic region.
I would expect three formal nights and perhaps three or four semiformal nights on a transit. These cruises might have a couple more dressy evenings than most due to the absense of ports of call that make dressy evenings impractical.