Congratulations and welcome to the cruising community!! You've made an excellent choice for your first cruise IMO. Weather will be warm, in fact you should have some very nice PM temps at this time unless it's rainy, etc. Besides 2 formal nights (the 1st full day/night at sea and second-to-last night), there will be an "informal" night at which time nice dress or pantsuit is ok, you'll see some people actually in more formal dress even on "informal" night, but do what you're comfortable with. For "casual" nights, nice slacks would be fine, personally I think I'd stay away from capris, but that's me, though there are some cute capri outfits that probaly would be ok. I usually wear casual cotton dresses or pantsuits or nice slacks with blouse for casual nights. Jeans and shorts are a no-no in the dining room and (thankfully) Celebrity does enforce this, other than breakfast and lunch, at which time no swinsuits or even suits with coverups are allowed. You can go casual in the buffet/Lido dining anytime. If your itinerary is same as last time I wnet West with Celeb, you'll get Key West which is one of my favorite places though you may not have much time there - if you're a cat lover you have to see Hemingways, the trolley tour is nice and I also like the Mel Fisher Museum. In Calica/Costa Maya do a ruins tour, Tulum is worth seeing, Xcaret park is also very nice; it will be very hot so take plenty of water. In Cozumel, if you snorkel you'll love this stop; not much in the way or ruins here, if you like to shop it's also pretty good for that. If there's time, do stop at my favorite place, Pancho's Backyard, for lunch and/or a margarita, they also have a one of my favorite gift shops. I'm cruising 11 days Hawaii to Ensenada on Mercury starting Oct. 10th - hope you enjoy yours as much as I know I'll enjoy mine. Post a review when you get back!