Unfortunately, being a guy, I can't tell a lady how to "dress up". However, I can offer some feedback...
Probably 25% of the men will be wearing a tux, the rest wearing dark suits.
A typical 7-day cruise has 2 formal nights.
Formal nights are never the first night, and never the last night.
I've never seen two formal nights in a row.
Lobster seems to always be served on a formal night...