Usually they are more relaxed on the 1st night becuase of luggage. So you can wear your casual clothes the 1st night.
Carnival Dream for Thanksgiving
Eastern Caribbean- 11/20/2010
Carnival Ecstasy (4 Day Baja Mexico) 2003
Carnival Pride (7 Day Mexican Riviera) 2007
Carnival Spirit (8 Day Exotic Mexican Riviera) 2007
Carnival Freedom (7 Day Western Caribbean) 2008
Carnival Elation (4 Day Baja Mexico) 2009
Carnival Spirit (7 Day Southbound Alaska) 2009
Not to start a war but... casual is in the eye of the beholder. I have been to formal nights where some one "dressed up" in a velor sweat suit. So I am guessing as long as you don't show up in a speedo or bikini they will let you in the door not matter what the "dress code" says.
Back to your question... the first night people show up in what ever because of luggage issues. Good question, because I didn't actually know this until my 3rd cruise. Enjoy!
Sensation (without KathieLee) July 2001
Paradise (smokefree) March 2002
Valor (inaugural season) May 2005
Truimph May 2007
Fascination (1st time w/DD) May 2008
Pride (with DD & DS) November 2009
They do tend to be a little more lenient the first night since some folks may not have received their luggage by dinner time. This did happen to us on our first cruise on the Celebration and I had to go to dinner in shorts. I was very embarrassed but noted that there were others like me. On our next cruises I was sure to wear slacks or something not quite so casual to board in. Lately, we have received our luggage quite early and have not had that problem. But to answer your question, yes, they are a little more relaxed on the first night.
S/S Yarmouth-7/62; Celebration - 9/99; Inspiration - 12/00; Jubilee - 10/02; Elation Thanksgiving cruise - 11/03; Elation again - 12/04; Carnival Conquest - 2/06; Carnival Glory Super Bowl cruise - 2/07; Ecstasy - 6/07; Ecstasy - 11/07; Carnival Conquest - 10/08; Conquest inaugural cruise to the Bahamas - 2/15/09; Triumph cruise to Western Caribbean Jan. 16, 2010, Ecstasy cruise to nowhere, July 4 weekend 2010. Carnival Magic, 10-28-2011, Barcelona to Galveston.