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Your question is so open ended I don't know where to start, but here goes. There are typically 1, 2 or 3 Formal Evenings depending on the length of the cruise. The other evenings will be Smart Casual. Hope this was the answer you required, if not click the comment button & respond.
At least a couple of weeks before you go on Sea Princess, you will be sent a pack with a booklet called 'cruise answwers' which details how the American company determines what casual/smart casual and formal mean in terms of dress. You will also be sent an itenery sheet confirming your dinner seating, cabin no. etc. and also how many nights will be formal, smart casual and casual. There are usually 2 formal per week. Everyone dresses up. The few who dont usually eat in Horizon court or the pizzareia. the choice is yours but everyone looks so lovely it is worth the effort and to be honest it is the only chance for many these days to get dressed up with all the bling on !!
We actually plan our cruises around being able to " dress up " - it is great fun and you actually get chance to have some lovely photographs taken - very nice if like me you are always the one taking the pictures.As the previous comments say you can opt out and eat in the buffet restaurant but my advice would be go for it - if you would like some information about obtaining long dresses I can recommend a good site on the internet.