Hi Aisling,
I can try to answer a few of your questions. On our 12/03 Christmas cruise, the formal night was Monday (12/22), and semi-formal was Thursday, Christmas night.
Perhaps because it was the height of the holiday season, everyone was dressed up on Formal night. Many women (all in our party) wore long formal gowns, and the majority of our daughters did also. There were also beautiful short cocktail dresses, and a lot of beading! There were many, many men in tuxedos. Everyone looked beautiful! The younger girls were precious in their party dresses, and there was a lot of picture-taking going on. Disney sets up staged photos in the atrium before each dinner seating for formal pictures to be taken of your family, which you can purchase later at Shutters. We have some beautiful pictures of our family, our girls, and our friends!
On semi-formal night (Christmas Dinner) I wore a silk pantsuit. The top was black, buttoned down the front and had 3/4 length sleeves. The pants were black with a red and gold floral print, with red beading on the red flowers. The pants were ankle length, and I wore black embroidered sling back pumps. My DD-17 wore a black dress with hot pink trim at the bodice and hot pink spaghetti straps, a dropped waist with a handkerchief hem. She is very tall and slender, and looked beautiful! My DD-13 wore a strapless black and white print dress. The bodice was ruched (I don't know if I spelled that correctly, but it was gathered from top to bottom, horizontally) and had a full skirt to about 4" above the knee. I think they both went without stockings, and wore black slides with heels.
My DH rented a tux from the site that
DCL suggests. There is a link somewhere to that site. They require tuxedo measurements when you fill out the order form online. Your DH (and DS's) can go to any tux rental store in your area, and tell them that they need to be measured for a tux to be rented out of town. I think most tux places do this free of charge. When you get the measurements, fill the order form out online and send it in. We did not have our cabin number (we had purchased late, like you, on a spur of the moment, and could only get a GTY status). My DH's tux was delivered to our cabin around 5:00 p.m. the first night onboard. BE SURE TO HAVE ALL OF YOUR MEN TRY THEIR ENTIRE OUTFIT ON as soon as they arrive. My DH's jacket was way to big, and he needed to get a smaller one. Although they have a good stock of pieces onboard, it is much easier to make sure you get everything that fits correctly Saturday afternoon, rather than waiting until the next morning, or, heaven forbid, an hour before dinner!
My DH has a tux, but had "grown out of it". He has several sets of ties and cumberbunds, which he packed in his suitcase. He switched them out on formal and semi-formal nights. Several gentlemen had Disney sets, which they wore on semi-formal night - fun! You can purchase tuxedos on Ebay that are close in price to what you might pay to rent, but when you add in the shirt, vest, shoes, suspenders, etc. and sending it out to be ironed onboard, you may find that renting is less expensive and easier, depending on whether your family members will have use for it down the road.
I brought a pair of long black slacks, a pair of black capris, and several Christmas sweaters that have Christmas designs and are beaded. I wore these on the other nights. I think I packed my black pants on the same hanger as the pantsuit, and the sweaters rolled up and packed easily in my suitcase. There are lots of ways to do the formal and semi-formal, a long black skirt with different tops, a long black dress with different accessories (scarves, etc), black slacks, etc.
Mexicali night was held the night we were in Cozumel. Besides our excursions, we did a little bit of shopping and the girls each got a colorful mexican outfit of a tank top and with matching wrap around long pants in very colorful mexican print fabrics. They were booked to go on the teen catamaran trip that evening. There were many women in mexican skirts, and the pretty white mexican blouses that are gathered at the neckline with embroidery. There were a lot of capris and colorful shirts, also. Men were in colorful "hawaiian" shirts, or mexican shirts, and there were a lot of shorts that night. Everyone is given a lei to wear at dinner.
There is a lot of discussion on the boards about conforming to the suggested dress code. Most people do, and there are those who feel that it is their vacation, they paid the same price as everyone else, would be more comfortable in less dressy attire, and so they choose to dress accordingly. I don't think I have seen a post on the board where someone has actually been turned arway from a dining room at dinner time because they were wearing shorts and a t-shirt.
I hope I am not giving you a headache with MY detail-oriented answers, but since it was my first cruise, I had absolutely no idea what to expect, so I went on the advice from the boards. I do tend to dress up more than dress down (except at home or at the beach - LOL), and figured for the price of the cruise, there would be plenty of other women doing the same for the holidays. I was right.
You are right, the planning is at least 50% of the enjoyment of these trips. Please feel free to ask and we will try to answer.
If I can figure out how to do it, I will scan in the photos of formal and semi-formal night, and put them on my Webshots site so you can see what everyone was wearing, and get an idea of the types of photos are taken.
Looking forward to meeting you!
Bonnie
We can't pass that up, so we're moving the date again.
Lastly, the tuxes are meant to fit a variety of folks so they have some built-in comfort measures............expandable waist straps, etc. 
They did bring us a big fan but I still felt like an egg on the sidewalk! 
