Yes, on my past 2 cruises, there has been a captain's reception on the same night as the formal night. As Peter stated, this is your chance to have your picture taken with the captain and meet some of his staff. Complimentary champagne for those 21 and over usually is served.
Formal attire is usually for the Captain's reception as well as dinner. After these events, you can go and change to casual attire.
2007 - Majesty of the Seas (Bahamas)
2009 - Enchantment of the Seas (Western Caribbean)
2010 - Freedom of the Seas (Western Caribbean)
2013 - Golden Princess (Alaska, roundtrip from Seattle)