Glad to see you are cruising again. It will depend on your itinerary, but generally the formal nights are the first sea day and the first return sea day. If the second day is before the last night of the cruise then the formal night wil be the day before.
All I ask is a tall ship and a star to steer her by; John Masefield
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014, Regal 2014, 2016, Royal 2017
Star Clippers: Royal Clipper 2015